Is timing really everything? Evaluating Resource Response to Spring Disturbance Flows

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Glen Canyon Dam has altered ecological processes of the Colorado River in Grand Canyon. Before the dam was built, the Colorado River experienced seasonable variable flow rates, including springtime flooding events. These spring floods scoured the river bottom and enhanced natural processes that sustained the Colorado River ecosystem. Since the dam’s construction in 1963, springtime floods have been extremely rare and of low magnitude. Now, after more than 50 years of operation, the apron (underwater portion) of the Glen Canyon Dam requires maintenance. To facilitate this, releases from Glen Canyon Dam will be reduced to 4000 cubic feet per second (cfs), which is about one-third the average for March. This low flow will be immediately followed by a high flow disturbance which will consist of water releases of about 20,150 cfs. This combination of low and high flow disturbance is expected to enhance the natural processes that sustain the Colorado River ecosystem by mimicking springtime pre-dam flooding.

Since 2012, five fall-timed High Flow Experiment (HFEs) have benefitted sand bar resources. However, the effects of spring-timed high flow experiments on achieving resource goals remains unknown. Thus, the spring disturbance flow hydrograph was designed collaboratively by stakeholders and scientists to fill this gap and potentially benefit resources.

Hydrograph

Normal dam operations in March release water between about 9,000 to 17,000 cfs. Beginning March 15, 2021, water released will be reduced to a steady 4,000 cfs for five days while maintenance is conducted on the apron of the Glen Canyon Dam. The low flows will be followed by a high flow event. Water releases will be ramped up for four and a half days to about 20,150 cfs. The high flow will remain in effect for 82 hours (3.5 days). The experimental flow will conclude on March 26, 2021, with dam operations returning to normal. The combination of the drying out at low flow followed by scouring due to the high flow is expected to disturb the riverbed and enhance natural processes to a much greater extent than either flow would do alone.

Graph showing amount of water released from Glen Canyon Dam March 2021

The Spring Disturbance Flow was designed to mimic natural spring flooding events prior to the construction and operation of the Glen Canyon Dam in 1963. The experimental flow is expected to revitalize the Colorado River ecosystem. USGS scientist will be monitoring natural resources response to the flow over the course of months to years. 

(Credit: GCMRC. Public domain.)

Natural Resource Concerns in Grand Canyon

Does Disturbance Timing Affect Food Base Response?

Primary Contact: Ted Kennedy (tkennedy@usgs.gov)

            Spring disturbance flows in other river systems have been shown to improve the health of invertebrate assemblages. In the absence of spring timed flood disturbance, nonnative and slow growing and species often dominate invertebrate assemblages, and fast-growing insect species become less abundant. We think this dynamic is playing out in Glen Canyon, with nonnative New Zealand mudsnails currently dominating assemblages owing to the absence of spring floods and aquatic insect taxa being relatively uncommon. We will be collecting daily measurements of invertebrate populations during the experimental hydrograph to identify the extent to which this flow exports New Zealand mudsnails and other non-native taxa and favors fast-growing aquatic insects over the long-term. Data collected during the spring disturbance flow experiment will be compared with long-term monitoring data that have been collected since 2006.  By comparing invertebrate population response at these time points to our long-term data set, we will be able to determine the effect of spring disturbance flows on the health of invertebrate assemblages and the prey base for fish in the Colorado River ecosystem.

Bank Erosion, Bed Sedimentation, and Channel Change in Western Grand Canyon

Primary Contact: Paul Grams (pgrams@usgs.gov)

The prolonged drought in the Colorado River Basin that began in 2000 has caused the level of Lake Mead to drop by more than 130 feet (See water level table from Bureau of Reclamation). As lake levels dropped, the Colorado River cut (incision) into the former lake bed sediments, leaving behind tall banks of sand and silt that continue to erode and widen the channel. Further incision is prevented by the emergence of a bedrock control that causes an unnavigable rapid at the upstream end of Lake Mead. Sediment eroding from the banks and sediment entering the reach from upstream now accumulates on the riverbed and makes boat navigation difficult and hazardous. In a study developed in cooperation with the Hualapai Tribe, whose river running operation is particularly impacted by the navigation problems, we will be measuring how the pulse flow affects river bed elevations in this reach that is located more than 285 miles downstream from Glen Canyon Dam

Aeolian Response to a Spring Disturbance Flow 

Primary Contacts: Joel Sankey (jsankey@usgs.gov) and Helen Fairley (hfairley@usgs.gov

Archeological sites are important cultural resources in Grand Canyon. Many archaeological sites are buried under, and thus protected in place by, sand dunes. High flows typically resupply dunes with sand indirectly by depositing sand on river sandbars which is then blown by wind from the sandbars to the sand dunes. Previous high flow experiments have primarily occurred in the fall, creating uncertainty of how spring high flow events may affect sand dunes and archeological sites. Moreover, extreme low flow events expose more sand in the river channel that might be blown to dunes by seasonally strong spring winds. This spring disturbance flow allows us to study wind-borne sand movement at low flow periods and how spring-timed high flows will affect sand dunes and archaeological sites.

Riparian Vegetation Physiological Response

Primary Contact: Emily Palmquist (epalmquist@usgs.gov)

Riverside (riparian) plants in Grand Canyon vary in their dependence on water and their drought tolerance. Therefore, individual flow scenarios can positively affect some riparian plant species while negatively affecting others. Plant species that are highly dependent on water (such as willow, sedges, and rushes) may be negatively affected by the low-flow portion of the disturbance event, but those effects are expected to be minimal. Conversely, the high-flow portion is expected to provide these plants water when growth is expected to be greatest. The effects of the combination of low- and high-flow are unknown. Therefore, we will be monitoring several physiological metrics before, during, and after the spring disturbance flow.

Mapping Aquatic Vegetation Response to a Spring Disturbance Flow

Primary Contact: Kim Dibble (kdibble@usgs.gov)

The combination of the low- and high-flows are expected to affect aquatic plants. First, the low-flow will likely dry out macroalgae, and vascular and non-vascular plants. Second, the high-flow will likely remove many of the dried out macroalgae, making room for microalgae (specifically, diatoms) to increase. We will use state-of-the-art mapping techniques to determine the aquatic plant communities before and after the spring disturbance flow.

Brown Trout Early Life Stage Response to a Spring Disturbance Flow

Primary Contact: Kim Dibble (kdibble@usgs.gov)

Brown trout peak emergence occurs during the first two weeks of March. Exact timing depends on water temperature. The spring disturbance flow will likely not affect spawning timing or success, as these occur prior to the scheduled spring disturbance flow. However, the timing of the spring disturbance flow will correspond to emergence of brown trout. We will monitor populations in Lees Ferry for the next two years and compare population levels to previous years without disturbance events.

Do Disturbance Flows Significantly Impact Recreational Experience?

Primary Contact: Lucas Bair (lbair@usgs.gov)

Recreationalists may be affected by the spring disturbance flow.  Some anglers might find river access more difficult during the low flow period. Likewise, whitewater rafters will have a more difficult time maneuvering during the low flow period, but will have more potential campsites, whereas the high flow portion will result in the opposite (fewer camping areas, but more rapids and easier navigability). We will be conducting surveys before, after, and during the spring disturbance flow to measure recreationalists, specifically anglers and whitewater rafters, are affected by this flow. Recreationalists will be interviewed on-site or sent a mail survey packet. This information will help inform decisions about tradeoffs that occur, with regard to recreation, when evaluating future higher spring flow experiments.

Sandbar and Campsite Response to Spring Disturbance Flow

Primary Contact: Paul Grams (pgrams@usgs.gov)

Most campsites that recreationists use while boating or hiking along the Colorado River in Grand Canyon are located on sandbars deposited by the river. Several high-flow experiments (HFEs) have been conducted to rebuild eroded sandbars and thereby increase the size and number of campsites. Because the peak flow portion of the pulse flow is less than half the magnitude of the typical HFE, only minor changes to the low-elevation parts of sandbars are expected. Those changes will be evaluated by analysis of daily images from a network of automated cameras.