Sediment transport, streamflow, and climate change: long-term resilience of the Bay-Delta
Sediment supply is important to the health of the Sacramento-San Joaquin River Delta and San Francisco Bay (Bay-Delta) ecosystem. Sediment eroded from upland source areas in the Sacramento and San Joaquin watersheds is transported through the rivers to the Bay-Delta where it is deposited in mudflats and tidal wetlands, which in turn helps protect against the effects of sea-level rise. Sediment deposits are also vital to the preservation of wetlands and habitat for wildlife such as birds and fish. Sediment supply from the Sacramento River has been declining since at least the mid-1950s, due to trapping of sediment in reservoirs and legacy effects of hydraulic mining, among other landscape disturbances. The future sediment supply to the Bay-Delta is primarily dependent on the future climate, as sediment supply is strongly dependent on peak river flows.
Coastal wetlands, including the Bay-Delta, are under direct and increasing threat from land use change pressures, from indirect impacts of upstream disruption to sediment supply, and from development pressures and rising sea level on the coastline (Syvitski et al., 2009). Sustainable management of coastal wetlands and marine ecosystems offer a wide range of ecosystem services, including shoreline protection, nutrient cycling, water quality maintenance, flood control, habitat for birds and other wildlife such as fish, and opportunities for recreation.
Over the past several decades, the health of the Bay-Delta has been in decline (Healey et al., 2008) due to a reduction in water and sediment supply, invasive species, toxic pollutants, land use changes, levee systems in the Delta, and land subsidence from the draining of wetlands. Suspended sediment concentration (SSC) is an important estuarine health indicator (Achete et al., 2017), and sediment supply to the Bay-Delta, which is derived from the upstream watershed, has been declining over the past half-century (Stern et al., 2016; Wright & Schoellhamer, 2004). Anthropogenic and natural changes to the watershed can contribute to a decline or increase in sediment supply, which in turn impacts primary production, fish habitat conditions, contaminant transport, marshlands, and protection against sea level rise.
Simulating Future Sediment Supply
The future of sediment supply to the Bay-Delta is largely unknown. Future assessments using climate and management scenarios are imperative to help manage the Bay‐Delta through natural and anthropogenic changes in sediment supply. The Computational Assessments of Scenarios of Change for the Delta Ecosystem (CASCaDE II) project is composed of a diverse set of interconnected models (global climate models, hydrodynamic, operational, marsh accretion, contaminants, and biological) that work together to quantify effects of climate change and management on the overall health of the San Francisco Bay‐Delta ecosystem.
As part of the CASCaDE II project, a daily rainfall, runoff, and transport model (Hydrological Simulation Program - FORTRAN (HSPF)) of the Sacramento River Basin of northern California was developed to simulate streamflow and suspended sediment transport to the Bay‐Delta for the next century (water years, WY2010–2099) using 20 future climate projections. While streamflow has clearly been affected by the anthropogenic development of the Sacramento River watershed, it is likely that a combination of other factors, such as hydraulic mining, dam construction (reservoir sedimentation), land use changes, agricultural practices, logging, and river engineering works (levees, navigation dredging, etc.), and changes in climate have had a greater influence on sediment supply than streamflow. Reservoir sedimentation has been determined to be one of the main factors in the decline in sediment supply over the recent decades (Wright & Schoellhamer, 2004).
The simulated projections indicate increased peak streamflow and increased sediment supply to the Bay‐Delta which would aid in marsh accretion and bolster the resilience of marshes against sea level rise. Maintaining marshes longer into the century sustains the habitat of many native fish and bird species that could otherwise be lost. Tidal flats and beaches in the San Francisco Bay rely on a constant sediment supply to be sustained, and the threshold rate of sea level rise resistance is typically dependent on sediment availability (Kirwan & Megonigal, 2013). Some fish species depend on turbid waters to reduce the risk of predation, as well as potential increased deposition of spawning gravels with very high flow events. Excessive fine‐grained sediment can clog spawning gravels for salmon. Too much sediment increases light attenuation and could have adverse effects on benthic organisms and thus the rest of the food web in the Bay‐Delta.
Projected water and sediment supply trends for the next 100 years will help resource and land managers prepare for the effects of climate change in a complex and highly managed estuarine system.
References
Achete, F., Van der Wegen, M., Roelvink, J. A., & Jaffe, B. (2017). How can climate change and engineered water conveyance affect sediment dynamics in the San Francisco Bay‐Delta system? Climatic Change, 142(3–4), 375–389. https://doi.org/10.1007/s10584-017-1954-8
Healey, M. C., Dettinger, M. D., & Norgaard, R. B. (2008). The state of Bay‐Delta Science, 2008 (pp. 174). Sacramento, CA: CALFED Science Program.
Kirwan, M. L., & Megonigal, J. P. (2013). Tidal wetland stability in the face of human impacts and sea‐level rise. Nature, 504(7478), 53– 60. https://doi.org/10.1038/nature12856
Stern, M. A., Flint, L. E., Minear, J., Flint, A. L., & Wright, S. A. (2016). Characterizing changes in streamflow and sediment supply in the Sacramento River basin, California, using hydrological simulation program—FORTRAN (HSPF). Water, 8(10) 432 p, 432. https://doi.org/10.3390/w8100432
Syvitski, J. P. M., Kettner, A. J., Overeem, I., Hutton, E. W., Hannon, M. T., Brakenridge, G. R., Day, J., Vörösmarty, C., Saito, Y., Giosan, L., & Nicholls, R. J. (2009). Sinking deltas due to human activities. Nature Geoscience, 2(10), 681– 686. https://doi.org/10.1038/ngeo629
Wright, S. A., & Schoellhamer, D. H. (2004). Trends in the sediment yield of the Sacramento River, California, 1957–2001. San Francisco Estuary and Watershed Science, 2(2). https://doi.org/10.15447/sfews.2004v2iss2art2
Sediment supply is important to the health of the Sacramento-San Joaquin River Delta and San Francisco Bay (Bay-Delta) ecosystem. Sediment eroded from upland source areas in the Sacramento and San Joaquin watersheds is transported through the rivers to the Bay-Delta where it is deposited in mudflats and tidal wetlands, which in turn helps protect against the effects of sea-level rise. Sediment deposits are also vital to the preservation of wetlands and habitat for wildlife such as birds and fish. Sediment supply from the Sacramento River has been declining since at least the mid-1950s, due to trapping of sediment in reservoirs and legacy effects of hydraulic mining, among other landscape disturbances. The future sediment supply to the Bay-Delta is primarily dependent on the future climate, as sediment supply is strongly dependent on peak river flows.
Coastal wetlands, including the Bay-Delta, are under direct and increasing threat from land use change pressures, from indirect impacts of upstream disruption to sediment supply, and from development pressures and rising sea level on the coastline (Syvitski et al., 2009). Sustainable management of coastal wetlands and marine ecosystems offer a wide range of ecosystem services, including shoreline protection, nutrient cycling, water quality maintenance, flood control, habitat for birds and other wildlife such as fish, and opportunities for recreation.
Over the past several decades, the health of the Bay-Delta has been in decline (Healey et al., 2008) due to a reduction in water and sediment supply, invasive species, toxic pollutants, land use changes, levee systems in the Delta, and land subsidence from the draining of wetlands. Suspended sediment concentration (SSC) is an important estuarine health indicator (Achete et al., 2017), and sediment supply to the Bay-Delta, which is derived from the upstream watershed, has been declining over the past half-century (Stern et al., 2016; Wright & Schoellhamer, 2004). Anthropogenic and natural changes to the watershed can contribute to a decline or increase in sediment supply, which in turn impacts primary production, fish habitat conditions, contaminant transport, marshlands, and protection against sea level rise.
Simulating Future Sediment Supply
The future of sediment supply to the Bay-Delta is largely unknown. Future assessments using climate and management scenarios are imperative to help manage the Bay‐Delta through natural and anthropogenic changes in sediment supply. The Computational Assessments of Scenarios of Change for the Delta Ecosystem (CASCaDE II) project is composed of a diverse set of interconnected models (global climate models, hydrodynamic, operational, marsh accretion, contaminants, and biological) that work together to quantify effects of climate change and management on the overall health of the San Francisco Bay‐Delta ecosystem.
As part of the CASCaDE II project, a daily rainfall, runoff, and transport model (Hydrological Simulation Program - FORTRAN (HSPF)) of the Sacramento River Basin of northern California was developed to simulate streamflow and suspended sediment transport to the Bay‐Delta for the next century (water years, WY2010–2099) using 20 future climate projections. While streamflow has clearly been affected by the anthropogenic development of the Sacramento River watershed, it is likely that a combination of other factors, such as hydraulic mining, dam construction (reservoir sedimentation), land use changes, agricultural practices, logging, and river engineering works (levees, navigation dredging, etc.), and changes in climate have had a greater influence on sediment supply than streamflow. Reservoir sedimentation has been determined to be one of the main factors in the decline in sediment supply over the recent decades (Wright & Schoellhamer, 2004).
The simulated projections indicate increased peak streamflow and increased sediment supply to the Bay‐Delta which would aid in marsh accretion and bolster the resilience of marshes against sea level rise. Maintaining marshes longer into the century sustains the habitat of many native fish and bird species that could otherwise be lost. Tidal flats and beaches in the San Francisco Bay rely on a constant sediment supply to be sustained, and the threshold rate of sea level rise resistance is typically dependent on sediment availability (Kirwan & Megonigal, 2013). Some fish species depend on turbid waters to reduce the risk of predation, as well as potential increased deposition of spawning gravels with very high flow events. Excessive fine‐grained sediment can clog spawning gravels for salmon. Too much sediment increases light attenuation and could have adverse effects on benthic organisms and thus the rest of the food web in the Bay‐Delta.
Projected water and sediment supply trends for the next 100 years will help resource and land managers prepare for the effects of climate change in a complex and highly managed estuarine system.
References
Achete, F., Van der Wegen, M., Roelvink, J. A., & Jaffe, B. (2017). How can climate change and engineered water conveyance affect sediment dynamics in the San Francisco Bay‐Delta system? Climatic Change, 142(3–4), 375–389. https://doi.org/10.1007/s10584-017-1954-8
Healey, M. C., Dettinger, M. D., & Norgaard, R. B. (2008). The state of Bay‐Delta Science, 2008 (pp. 174). Sacramento, CA: CALFED Science Program.
Kirwan, M. L., & Megonigal, J. P. (2013). Tidal wetland stability in the face of human impacts and sea‐level rise. Nature, 504(7478), 53– 60. https://doi.org/10.1038/nature12856
Stern, M. A., Flint, L. E., Minear, J., Flint, A. L., & Wright, S. A. (2016). Characterizing changes in streamflow and sediment supply in the Sacramento River basin, California, using hydrological simulation program—FORTRAN (HSPF). Water, 8(10) 432 p, 432. https://doi.org/10.3390/w8100432
Syvitski, J. P. M., Kettner, A. J., Overeem, I., Hutton, E. W., Hannon, M. T., Brakenridge, G. R., Day, J., Vörösmarty, C., Saito, Y., Giosan, L., & Nicholls, R. J. (2009). Sinking deltas due to human activities. Nature Geoscience, 2(10), 681– 686. https://doi.org/10.1038/ngeo629
Wright, S. A., & Schoellhamer, D. H. (2004). Trends in the sediment yield of the Sacramento River, California, 1957–2001. San Francisco Estuary and Watershed Science, 2(2). https://doi.org/10.15447/sfews.2004v2iss2art2