The Hidden Sediment Reserve

Quantifying Potential Sediment Trapped Within the San Francisco Bay-Delta Watershed's Reservoirs

The past 150 years of human impacts on sediment flows in the Bay-Delta presents a fascinating and well-documented history. First, hydraulic gold mining and settlement-driven clearcutting sent an unprecedented pulse of sediment into Bay-Delta. Then, multiple dams throughout the upland portions of the watershed began to capture sediment while a variety of other infrastructures, such as levees and storm channels redirected sediment away from where they are needed. Today, rising sea levels threaten to wash away vulnerable wetlands and breach critical levees.

The sediment sources and sinks of the San Francisco Bay watershed have been dramatically altered over time. Hydraulic mining resulted in huge surplus of sediment in the watershed. In contrast, today we face a deficit due to widespread dam construction.

Scientists estimate that the Bay-Delta’s wetlands “would require a total sediment input (i.e. organic matter and inorganic sediment) of up to 10.1 Mm3/year (~2.6cm/year) by 2100 to keep pace with the higher projections of sea level rise; presently only as much as 0.4 Mm3/year is actually being deposited.” The Bay-Delta’s wetlands require 10.1 million m3 of sediment per year to keep pace with sea level rise. The watershed holds a bountiful reserve of trapped sediment that might help meet this need, hidden within its numerous dam reservoirs

Moving and utilizing this sediment for the Bay-Delta would face multiple challenges. The massive quantities of sediment would need to be physically transported downstream, through means such as dredging and trucking, dam retrofits, or deliberate flooding induced through dam management. Sediments vary greatly, and silts, sands, and gravels would all need to be matched to the specific locations where they could be used. Moreover, some sediment is contaminated by toxins and pollutants, including mercury from the hydraulic gold mining era.  

But these challenges are not fixed. Though mobilizing this sediment may seem far-fetched today, the calculus will shift as sea level rises, sediment shortages manifest, and dams exceed their lifespans.

The following interactive visualization encourages the user to explore the sediment volumes trapped within California’s constellation of reservoirs. Sediment composes the material infrastructure that supports the Bay-Delta’s many ecologies and economies. As the 2015 State of the Estuary report by The San Francisco Estuary Partnership notes, “Like freshwater, sediment is a precious resource that is essential for keeping the Estuary healthy." Navigate the map, select reservoirs, and scrub the timeline to gain insight into the quantities and equivalent values of the San Francisco Bay-Delta’s displaced sediment.

This project was created in collaboration with The Dredge Research Collaborative.

Click here to view the project