Sea-Level Rise, El Niño, and Storm Effects on Coastal Tidal Marshes

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Tidal marshes exist as a transitional environment between the land and sea, and provides habitat to fish and wildlife, protects human developments from coastal flooding, and stores carbon at high density, among other important ecosystem services. Over the next century, accelerating sea-level rise poses a risk to tidal marshes, especially in areas with insufficient sediment delivery. In the near-term, El Niño events and extreme storms can both increase and decrease marsh resilience to sea-level rise. Marsh wildlife inhabitants, including several endangered species, are at acute risk via increased predation and losses of both breeding area and high-tide refugia. Through a combination of long-term monitoring and computer simulations, this research explores the sensitivity of tidal marsh habitat to both short-term climate variation and long-term change and provide land managers the information needed to guide resource management and restoration practices. In this webinar, Southwest CASC supported researcher Kevin Buffington discusses how climate change may affect El Niño events and extreme storms on coastal wetlands.


Date Taken:

Length: 00:40:40

Location Taken: US

Video Credits

Kevin Buffington, Elda Varela Minder


Elda Varela Minder:  [00:05] Kevin Buffington is with the USGS Western Ecological Research Center. He joined the USGS in 2010 where he has been studying how climate change will impact tidal marsh ecology using a combination of long‑term monitoring, continued modeling, and remote sensing.

[00:21] He received his PhD in wildlife science from Oregon State University in 2017 with a focus on Pacific Northwest tidal marshes and their relative vulnerability to sea‑level rise. His research interests have expanded beyond tidal marshes to include mangrove ecosystems and modeling blue carbon.

[00:37] Welcome, and thank you so much for joining us in this NCASC series.

Kevin Buffington:  [00:39] Thanks for that introduction. Thanks for the Southwest Climate Science Center for funding this research, and for the National CASC for organizing this webinar. Thanks everyone for joining me here this afternoon.

[00:54] I'm going to be talking about research that is focused on Pacific Coast ecosystems, their vulnerability to sea‑level rise, and how periodic events such as storms and El Niño may influence their long‑term vulnerability to sea‑level rise.

[01:10] I'm going to start with a quick background on climate change, El Niño, atmospheric rivers, as well as coastal processes, before going through four case studies. First, I'll show projections of marsh elevation under accelerating sea‑level rise.

[01:27] Then I'll describe a historic El Niño event in 2015 and how that affected flooding in marshes across the Pacific Coast. After that, I'll talk about how a series of atmospheric rivers hit San Francisco Bay area in 2017 and how that affected marsh accretion processes. Finally, I'll close with a study that addressed how tidal marsh predators respond to marsh flooding.

[01:58] As you're probably aware, estuaries are incredibly dynamic ‑‑ subject to daily flooding by tides, seasonal variation in freshwater, and periodic events like storms and drought.

[02:10] Over the next century, climate change will alter estuaries through accelerating sea‑level rise, changes in freshwater availability, which in turn affects the salinity profile of an estuary. Increases in ocean temperature and acidification will have direct effects on fish and wildlife populations, while more frequent or intense extreme events may compound or mitigate the long‑term impacts of climate change.

[02:36] This is probably familiar to everyone. Human‑induced climate change has increased global temperature about 1.5C over the last 250 years, with a 0.9C degree increase just in the last 50 years. Oceans are absorbing the majority of this excess energy that is being trapped by greenhouse gases. More than 90 percent of the warming over the last 50 years has been occurring in the oceans.

[03:13] Without drastic changes in our carbon emissions, we will remain on the high end of climate projections, with an additional 4C or 10 degrees Fahrenheit of warming possible by the end of the century. This increase in temperature will bring with it more extreme and unpredictable weather and the possibility of prolonged drought conditions.

[03:34] As a result of the ocean warming and some melting of the polar ice caps, sea levels have risen about 20 centimeters or 8 inches since 1900. By the end of the century, between 40 and 100 centimeters of global sea‑level rise is likely, with some studies projecting upwards of two meters of sea‑level rise.

[04:04] Ocean currents and gravitational effects and vertical land movement all cause substantial variation in regional sea‑level rise projections. It's important to consider how sea levels are impacting your specific area.

[04:22] Now, the future is highly uncertain, especially when it comes to projections of sea‑level rise. If the Antarctic or Greenland ice sheet prove to be more unstable than previously thought, sea‑level rise could accelerate rapidly and result in worst‑case scenarios of over two meters of rise by 2100. Antarctica alone has a potential to contribute more than a meter of sea‑level rise by 2100, and more than 15 meters by 2500 if emissions continue unabated.

[04:59] Climate change is producing relatively slow changes in sea levels when compared to periodic events such as El Niño. El Niño temporarily elevates sea levels several centimeters over the course of a couple months, typically in the winter. El Niño is the result of complex interactions between the atmosphere and the ocean that generally cause elevated sea surface temperatures near the equator.

[05:27] El Niño comes in two distinct flavors, the eastern Pacific and the central Pacific, named for the regions of the elevated sea surface temperature when they occur. While both types increase temperature and ocean water levels, the eastern Pacific El Niños tend to have much more intense impacts along the US Pacific Coast.

[05:53] Our recent study examined signatures in coral reefs over the last 400 years and found that El Niño anomalies in the eastern Pacific have declined in the last 30 years, while El Niños in the central Pacific have increased in frequency. However, these eastern Pacific El Niños have grown much more intense over this time.

[06:14] The memorable El Niño events in 1982 and 1997 developed in the eastern Pacific. Climate change may be responsible for this shift in less frequent, but more intense, eastern Pacific El Niños that began around 1980.

[06:36] Switching to storms, atmospheric rivers are winter storm events that bring warm, wet air from the Tropics to the Pacific Coast. These events are responsible for delivering between 25 and 50 percent of the annual precipitation to California.

[06:52] Using an analysis of global climate models, a recent study found that climate change may result in a 45 percent increase in the frequency of these atmospheric river events along the western US coast as well as a 30 percent increase in the amount of moisture that these events deliver.

[07:14] Altogether the trends in sea level, El Niño, and Pacific storms point to a future with more coastal flooding along the Pacific Coast. To understand how tidal marshes may be affected, we first need to understand a bit about the processes that allow marshes to track sea‑level rise.

[07:38] Marsh elevation is controlled by the biologic and geophysical processes. Mineral sediment deposition is really important for increasing elevation. Tidal flooding delivers suspended sediment to the marsh platform with more flooding and higher sediment concentrations leading to more deposition and, ultimately, more elevation gain for accretion.

[07:58] Plant production results in the deposition of organic matter into marsh soils, which increases marsh elevation. The elevation of the marsh and the salinity of the water controls which plant species are able to survive and limits their productivity.

[08:18] Organic decomposition and compaction of the soil leads to elevation declines. The rate of marsh accretion is also related to the rate of sea‑level rise. As sea levels and flooding increases, more sediment can be deposited on the marsh, potentially offsetting that increase in sea level.

[08:40] Tidal marshes have existed for the past 5,000 years and have adapted to changes in sea level using these processes. However, at some point, the rate of sea‑level rise would be too great and the marsh will convert to an intertidal mudflat.

[08:54] Extreme events that affect these processes can have both positive or negative effects on marsh vulnerability to sea‑level rise. For example, extended drought conditions can stress marsh plants from increases in the salinity in the soil, leading to reduced productivity and lower organic matter deposition as well as less sediment deposition from upland erosion due to rainfall.

[09:21] On the other hand, increased upland erosion during large storms and heavy rainfall can deliver a pulse of sediment to the estuary that leads to an increase in marsh elevation.

[09:37] With that brief background, I'm going to dive into the first case study, which addressed this question, "How will tidal marsh elevation respond to sea‑level rise?"

[09:46] We examined marsh elevation responses to sea level across 14 study sites distributed along the Pacific Coast. Projections of sea‑level rise differ depending on where you are along the coast due to plate tectonics. There's tectonic uplift happening north of Cape Mendocino offsetting some of the rates in sea‑level rise.

[10:15] This figure shows the location of study sites, where we collected and dated soil cores using cesium radioisotopes and calculated accretion rates. The relative size of the circle reflects the average accretion rate at that site. Accretion rates were generally higher in the Pacific Northwest, but there is substantial variation along the coast.

[10:38] To characterize the baseline conditions at each marsh, we collected thousands of elevation points, conducted vegetation surveys, and monitored water levels to calculate local tidal datums.

[10:55] We then used the mineral and organic matter accumulation rates from those 36 soil cores to calibrate the WARMER model, and made projections under the three sea‑level rise scenarios which were on the previous slide.

[11:11] The accumulation rates from the soil cores reflect the climatic environment that the marsh experienced over the last 50 years or so. This includes any storm or El Niño events that may have occurred during that time. These accumulation rates don't account for potential changes in the frequency or intensity of extreme events.

[11:33] A bit more background on WARMER. WARMER is a process‑based model of the wetlands soil column. It considers the dominant above‑ and below‑ground processes that govern marsh elevation, including mineral and organic matter deposition, compaction, and decomposition.

[11:49] It's a one‑dimensional model just of the soil column, and so it doesn't consider potential for marsh migration upslope or erosion at the marsh edge. However, most marshes along the Pacific Coast are limited to the margins of the estuary by either human development, such as levies or infrastructure or by steep slopes. This more simplistic modeling approach for marsh elevation is somewhat justified.

[12:17] We'll dive right into these results, for the Pacific Northwest first showing that moderate sea‑level rise scenario, 63 centimeters by 2100. We characterize each marsh by the proportion of different vegetation communities. Dark green is high marsh, light green is mid marsh, the lightest green's low, and orange is mudflat.

[12:47] The x axis is divided into 10‑year increments, and changes of habitat reflect a marsh response to sea‑level rise. Over the first 50 years or so under these scenarios, marshes across the Northwest are projected to maintain or even increase their elevation, which is reflected in the increase in high marsh.

[13:07] After 2050, when the rate of sea‑level rise is projected to increase, going towards exponential, marshes begin to lose elevation relative to sea‑level rise, and marshes transition to mid‑ or low‑marsh habitat.

[13:25] Under the high‑sea‑level‑rise scenario, Pacific Northwest marshes maintain or, again, increase relative elevation until about 2050, after which that increase in sea‑level rise leads to more low marsh and eventual transition to intertidal mudflat.

[13:42] Across California, under a slightly higher moderate sea‑level rise, again, owing to the differences in plate tectonics, all these sites lost mid‑ and high‑marsh habitat, with most sites transitioning to at least 50 percent mudflat after a hundred years.

[14:02] Under high‑sea level‑rise rates, most sites begin transitioning to mudflat about 2050 or 2060, with no marsh habitat projected to remain after a hundred years.

[14:13] Again, the differences in both the sea‑level rise scenario and the historic accretion rates are responsible for the higher relative vulnerability of these California marshes compared with those in the Pacific Northwest.

[14:28] Again, these projections only incorporate the effects of sea‑level rise, and it is possible that more intense or more frequent extreme events could offset some of these losses.

[14:48] Brief summary here. Using accretion rates based on historic conditions, most Pacific Coast marshes are projected to be lost in those high‑sea‑level‑rise scenarios. Moderate sea‑level rise leads to some marsh loss and a transition from high to low‑ or mid‑marsh vegetation.

[15:07] However, there is substantial variation along the Coast in the timing of these transitions and ultimate vulnerability to sea‑level rise. Estuaries with a low sediment load are especially vulnerable to accelerating sea‑level rise.

[15:25] Now that we understand the vulnerability of these marshes to sea‑level rise under historic conditions, we can start to address how periodic events such as El Niño may affect marsh vulnerability. The goal of this study was to examine the historic 2015 El Niño events for flooding events along the Pacific Coast.

[15:46] We used water level data across 10 marshes distributed all across the Coast to generate a offset model for water level, compared to the nearest NOAA tide gauge. We then applied this offset model to NOAA data going back searching from 1990 to 2016. For an analytical comparison of the 2015 El Niño, we split this period into pre‑, early, full, and late El Niño periods, based on sea surface temperature anomalies as well as some limitations in the water level data.

[16:28] With this marsh‑specific water level model, we calculated the difference between observed water level and the level predicted by tidal harmonics, the water level anomaly. We compared that anomaly then across the Coast.

[16:51] Using satellite data from NASA, we mapped out the open ocean conditions throughout the El Niño periods. The top row of figures is the sea surface temperature anomaly for each period, SSE, and then the bottom row is the sea surface height anomaly.

[17:11] We see that sea levels are elevated up to 24 centimeters during the full El Niño period just off the coast of Washington, but that there was also a lot of spatial variation as well as temporal variation along the coast.

[17:36] Those are the differences in the open ocean. How do those differences translate to flooding at our existing marsh sites? Along the Coast, we did find differences in high‑tide anomalies during the full El Niño and the late El Niño periods. During the full El Niño period, there was additional 15 centimeters of flooding at Coos Bay which is located in Southern Oregon.

[18:05] There was only about an additional four centimeters of flooding at Morro Bay in Central California. This additional flooding could have implications for both the wildlife that live in these marshes as well as for marsh processes, including sediment delivery and soil salinity.

[18:23] When we're looking back at the last 26 years, since 1990, the 2015 El Niño had some of the largest sea surface temperature anomalies and high‑tide flooding anomalies in the data set. Each dot on this figure, which is quite small probably, is the average monthly water level anomaly since 1990.

[18:50] The red dots, you get the full El Niño period during the 2015 event. The slope of these regressions now, is fairly consistent with a three to five centimeter increase in water level for each degree in sea surface temperature.

[19:14] Across the Pacific Coast, the 2015 El Niño elevated marsh high‑tide flooding by between 4 and 15 centimeters. The increase in sea surface temperature during the El Niño was related to these higher water levels.

[19:30] On the coast, El Niño affects water levels during the winter. There may not be strong direct affects on marsh plant productivity. However, additional flooding may flush salt from the soil, reducing soil salinity and could result in increased production the following summer.

[19:49] Additional flooding could also deliver more sediment to the marsh, increasing marsh accretion. However, now this is contingent on there being more sufficient supply of sediment in the estuary.

[20:02] Wildlife that lives in the Pacific tidal marshes, which is the salt marsh harvest mouse or the Ridgway's rail may experience additional mortality due to drowning or predation during periods of elevated flooding.

[20:13] Moving on to the extreme storm events that hit the San Francisco Bay region on February, 2017, with this study, we leveraged long‑term monitoring plots to examine how this event affected sediment accretion and marsh elevation throughout the San Francisco Bay estuary.

[20:41] Well, first, I'll provide a little bit of context. California receives over half of its annual rainfall in the form of atmospheric rivers. However, in 2014, there was a patch of high pressure off of the Pacific Northwest Coast, known as The Blob, which blocked atmospheric rivers from reaching California which resulted in an intense multi‑year drought.

[21:08] By late 2016, The Blob of high pressure started to weaken, opening the way for atmospheric rivers to return to California. Indeed, in early 2017, a series of atmospheric rivers hit the Bay Area.

[21:27] These storms caused tremendous flooding and damage, including the failure of the Oroville Dam spillway, which lots of you remember, putting thousands of people at risk of catastrophic flooding.

[21:38] For the course of log year 2017, over 42 inches of rain fell over the Bay Area, about two and a half times the annual rainfall in 2016 or in 2018.

[22:00] To examine the effects of the tidal marshes, we used a transect of five sites to show how this storm might have impacted marsh accretion rates. The Miner Slough is a freshwater tidal marsh located furthest upstream along the Sacramento River.

[22:18] Browns Island is located at the confluence of the Sacramento and San Joaquin rivers and is dominated by brackish plants.

[22:27] Rush Ranch is located at the north end of Suisun Bay. Out of the direct line of the flow from the Delta now, but it still receives a lot of freshwater from there. San Pablo, the north shore of San Pablo Bay, and the Pablo Marsh is located up the Petaluma River. Both of these sites are characterized by high marsh platforms and dominated by pickleweed.

[22:56] We have multiple surface elevation tables or sets installed at each site, which we read every three months over the study period. We also have a water level logger deployed near each set to calculate the average water levels and inundation rates. We focused on the winter time period for each of these three years, 2016, 2017, and 2018.

[23:19] If you're not familiar, surface elevation tables are used to monitor millimeter scale changes in marsh elevation through time by taking repeated measurements at an established benchmark. A lot is driven down to a point of refusal and provides a stable benchmark from which measurements can be made.

[23:48] Soil plugs are samples from a feldspar horizon plot to measure the depth of surface deposition. Between the pin readings which get at total elevation change, a net elevation change, and feldspar, which measures surface deposition, we get a good idea of the relative contributions, acute elevation change in marshes.

[24:20] First, the flooding, the water level results, in winter 2017, there was a substantial anomaly in the water level as a result of the atmospheric rivers. Flooding and anomalies followed gradient from upstream, starting at Miner Slough down to the Golden Gate gauge, which is off the map here on this gradient.

[24:48] Which in 2016 sand 2018, there was essentially very little anomaly across these sites. That previous graph, that was showing the average tide height, this is showing just the low‑tide and a high‑tide anomaly where we see that 2017 atmospheric river elevated both low and high tides in this period, especially evident at Miner Slough.

[25:20] Using a digital elevation model and water level data, we found that flooding in 2017 at Miner Slough...

[25:27] [audio cuts out]

Kevin:  [25:27] with a While Rush Ranch and Petaluma are both high‑elevation marsh platforms which is probably some of that difference.

[25:54] This figure shows the cumulative elevation change through time from the SETs through the line for Petaluma in red and Marsh Ranch in gray. They didn't really experience any lasting elevation change due to the storm, but they experienced elevation change that indicates seasonal dewatering and swelling.

[26:18] Elevation tends to go up in spring and fall in the late summer due to lack of precipitation, but there is a general upward trend.

[26:32] At San Pablo Bay, there was an increase in elevation, but it was delayed until the July readings. The storms occurred roughly in here. We had a reading in late March and another one in July. We didn't pick up the increase in elevation until July.

[26:52] What we suspect might be happening is that the storm delivered a bunch of sediment to a nearby mudflat that's adjacent to San Pablo Marsh, and then the sediment was resuspended by wind waves later in the spring for deposition on the marsh, perhaps during a king tide cycle in June.

[27:13] Both Browns Island and Miner Slough experienced elevation gains at what could be directly attributed to the storm. These sites are again relatively upstream and directly aligned with flow from the Sacramento River.

[27:33] Browns Island was increased by about 20 millimeters while Miner Slough increased elevation by 46 millimeters.

[27:40] After this initial pulse of elevation, both sites did decrease a bit in elevation due to compaction and dewatering that occurs. In general, the marsh elevation was maintained through the spring of 2018, which is the end of our data set here.

[28:10] The top figure shows feldspar accretion on the surface deposition across the five sites for each of the three years. The bottom graph is that net elevation change.

[28:28] You see that they don't necessarily correlate, which indicates the importance of this belowground processes, and controlling short‑term changes in marsh elevation.

[28:43] Across all the sites in those three years, longer inundation time was correlated with higher rates of feldspar or surface deposition and also highlights the importance of sediment delivery to long‑term marsh persistence under sea‑level rise.

[29:05] Apparently, San Francisco Bay is experiencing just under two millimeters a year sea‑level rise. All of our sites appear to be keeping pace with that level of rise. San Pablo Bay and Rush Ranch do have average elevation change rates that are slightly below that long‑term rate.

[29:23] However, the confidence intervals overlap that long‑term rate. We can say that they are keeping pace though.

[29:31] Assuming this same linear rate as sea‑level rise, the storm deposition at Miner Slough was enough to offset roughly 13 years of sea‑level rise. While the deposition at Browns Island offset roughly three years of sea‑level rise. There's a substantial amount of deposition.

[29:49] Now to place those accretion rates and general trends in better context, here are some set results for seven other estuaries across California. Of the 16 marshes that we monitored, 12 have relative accretion rates that have confidence intervals at over zero.

[30:11] Here, on the Y‑axis is the rate of change relative to sea‑level rise. This is times for the long‑term rate of sea‑level rise as well as vertical land movement and the sea level elevation change relative to that.

[30:28] Marshes in Humboldt Bay, especially, seem to be vulnerable. They have an accretion deficit of roughly two and a half millimeters a year. Sites do appear to be keeping pace with sea‑level rise.

[30:51] We found that flooding was elevated during the year, with multiple atmospheric rivers. As a result of the storms, [indecipherable] elevation increased at some sites with the highest deposition occurring at a site that was furthest upstream.

[31:07] The pulse of sediment effectively offset around 13 years of linear sea‑level rise at Miner Slough. There was a delay in deposition at San Pablo, possibly due to wind‑wave resuspension of sediment on nearby mudflats.

[31:24] Wildlife species that inhabit these marshes are used to regular flooding. However, prolonged flooding that was experienced in 2017 could have impacted their populations.

[31:42] Lastly, I wanted to show results from a study that we conducted to understand the behavior of avian predators and how that behavior changes relative to water level.

[31:54] Sea‑level rise, El Niño, and storm events all result in an increase in flooding across tidal marshes. For wildlife that live in these marshes, tidal flooding is a daily occurrence that they have to contend with. Some species move to higher ground during flood tides, while others simply perch on taller vegetation and wait for the water levels to subside.

[32:16] Both of these behaviors do increase their exposure to predators. What our question was, is does changes in water level make them potentially more vulnerable to predators? Now, marsh species are extremely difficult to study. They're usually secretive and hard to spot. We focused on the predator response.

[32:42] To test this question, we observed avian predator behavior at four tidal marsh sites across San Francisco Bay. Black John is located along the Petaluma River. Tolay Slough is on the north shore of San Pablo Bay, while China Camp is on the southwest shore. Arrowhead is down here in central San Francisco Bay, in urban area probably near the Oakland Airport.

[33:12] From an elevated vantage point, we surveyed each site 12 times from September to January, alternating between high and low daytime tides. For each survey, we alternated between focal observations and scanned surveys.

[33:32] During the focal observations, we recorded the activity of a single individual for about 12 minutes. Then during the scans, we recorded the abundance of all avian predators that we could see in the marsh.

[33:49] We grouped the predators into three guilds ‑‑ Ardeids, raptors, and scavengers. The most common Ardeids were great and snowy egrets as well as baby blue herons. The most common raptors were red‑tailed hawks, white‑tailed kites, and northern harriers, while the most common scavengers were gulls, crows, and turkey vultures.

[34:21] We found that the relative abundance of the predator guilds was roughly equal between low‑ and high‑tide periods for each site. However, the composition of the predators was different. At Arrowhead, the community was almost entirely comprised of scavengers ‑‑ gulls, mostly ‑‑ while Ardeids were the majority at China Camp.

[34:41] Raptors comprised almost a third of the predators at Black John and Tolay Slough, potentially owing to the location of agricultural fields near both of those sites.

[34:53] From the focal surveys, we found that predation effort and success did vary spatially across sites and with tide height, although the differences were not significant due to the low number of predation events that we observed.

[35:12] Naturally, Ardeids' predation success tended to be greater during high tides...

[35:17] [audio cuts out]

Kevin:  [35:21] and raptor strikes also tended to be higher during high tides. The effect at the rest of the sites weren't significant.

[35:34] Using the scan data, we developed a statistical model for the presence of predators that controlled for season, time of day, tide stage, and water level relative to average marsh elevation. We found that there was significantly more raptors and Ardeids present during the high tide.

[35:53] The probability of Ardeids' presence, which we called here predation pressure probability, had increased linearly as water levels on the marsh increased, while the raptors had a peak occurrence when water levels were about 50 centimeters above the average marsh elevation.

[36:13] From this study, we found significant evidence that tidal marsh predators use water levels as a behavioral cue to initiate hunting in tidal marshes. Given these results and the concentration of threatened and endangered species that live in San Francisco Bay tidal marshes, elevated flooding during El Niño and atmospheric river storms are likely to increase the risk of predation and could have serious consequences on their long‑term population viability.

[36:44] However, additional studies are needed to investigate how extreme events can impact tidal marsh wildlife populations.

[36:56] Let me bring this all together. Using the historic rates of accretion from the soil cores, we found that Pacific Coast tide marshes are vulnerable to higher rates of sea‑level rise that are possible under those elevated emissions scenarios, especially later in the century.

[37:15] Historic El Niño events in 2015 caused elevated water level across Pacific Coast marshes with significant spatial variation. Large storm events such as atmospheric rivers can result in substantial sediment deposition that is important for offsetting relative sea‑level rise.

[37:33] However, within an estuary, that impact can be variable and can built on the flooding levels and the intensity or the amount of available sediment. The increase in atmospheric river frequency and intensity with climate change could offset the effects of sea‑level rise in some estuaries. However, the storms can also increase erosion.

[38:00] Finally, elevated marsh water levels impact wildlife populations through an increase in predation risk. We are continuing to study the combined effects of sea‑level rise and extreme events on coastal ecosystems and are developing a next‑generation model for large responses to climate change that integrates both the short‑ and long‑term climate impacts that affects marsh elevation and wildlife responses.

[38:33] With that, I'd like to thank the Southwest ‑‑ to CASC again for funding this work as well as the USGS WERC Techs. Special thanks to all the technicians who spent so much time out in the field collecting this data over the last 10 years or so. With that, I can address any questions that you guys might have. Thanks.

John Ossanna:  [38:55] Thank you, Kevin. Looks like we got a question come in. It's from June. Under SLR, where California shows a transition to mudflats, is there any possibility for shoreward movement of marsh?

Kevin:  [39:14] That's a great question. That is something we looked at as part of that study. I can't get into it too much, but really, unfortunately not a whole lot of space for upland migration. I think the majority of the area is either covered by human infrastructure or pretty steep slopes. There is unfortunately not a ton of accommodation space.

[39:41] I think there's some exception to that. I know the Tijuana River Estuary has quite a bit of space for upslope transition. Certainly certain areas in San Francisco Bay that are maybe currently agricultural lands, and there may be some potential there.

[40:01] A lot of especially the outer‑coast estuaries, those are in relatively constricted estuaries where the slope is pretty steep on top of the human infrastructure. Assuming we protect what is there, there's probably not a ton of room for upslope migration.

John:  [40:23] With that, I'd like to thank again Kevin and Elda for USGS continued support of this webinar series.