A common lament is that water is wasted when it flows out to the sea rather than put to use irrigating crops or supplying water to cities. But when rivers flow to the sea the water brings benefits to people and ecosystems that are rarely acknowledged. We asked Jim Cloern―a scientist with the US Geological Survey and a member of the PPIC Water Policy Center research network—to explain.

PPIC: What are some benefits that river’s provide when they make it to the sea?

Jim Cloern: Runoff from rivers brings many benefits to coastal communities, the Delta, and wetlands. For example, if you live in or around the Delta, river flows repel saltwater moving upstream. If the flow is too low, water in the Delta becomes too salty for growing crops or drinking.

Rivers also carry sediment, which is really important to the San Francisco Bay ecosystem, especially for sustaining tidal flats and marshes. We’ve invested hundreds of millions of dollars to convert salt ponds back to wetlands. Collectively, the restoration of wetlands in the Bay Delta is the largest such effort west of Rockies. We’ve breeched levees and seen these areas become colonized by wetland plants and transformed into habitats for birds and fish. The soils that form the base for these habitats have a natural inclination to sink and need continuous replenishment. Rivers also carry sand beyond the bay to the ocean, which is essential to keep California’s beaches intact. Without river sediment our beaches would disappear from natural erosion.

A third benefit is to the Bay-Delta. San Francisco Bay is an estuary that sustains plant and animal communities not found in other ecosystems. These communities are an important part of California’s remarkable biodiversity. We’ve learned from other estuaries around the world that these communities can disappear if river inflow falls below levels required to sustain them.

River flows also flush pollutants out of the bay. San Francisco Bay has been called an urbanized estuary. Sewage effluents, industries, and urban runoff carry nutrients, toxic pollutants, pharmaceuticals, and micro-plastics into the bay. Large river flows like we’ve seen this year dilute and carry those contaminants out of the bay.

PPIC: What might happen if more water is diverted from flowing to the sea?

JC: The essential question for managing the Delta is, what is an appropriate amount to divert? It’s question #1 for the State Water Board. If we divert more, particularly during dry periods, we run the risk of increasing salinity of the Delta and the bay. The bay could become saltier than the ocean—this happened in the 1976–77 drought. A hypersaline bay would affect what kinds of organisms could live in it, including microorganisms―the largest component of life in the bay. We don’t know a lot about them, but we do know that some microorganisms provide really valuable services to us. For example, bacteria in bay sediments convert nitrogen from agricultural runoff and sewage into nitrogen gas, removing an important pollutant out of the water. This process is used in some sewage treatment plants. We don’t know how increased salinity would affect these “pollutant scrubbers.”

We also have to think in terms of how higher temperatures in a warming world would interact with lower flows. This combination, coupled with nutrient pollution, could be a perfect storm for generating harmful algal blooms. We also need to consider the potential loss of new wetlands we have invested so much to restore―they might not be sustainable if river sediment flows are reduced at a time when seas are rising. And how might the function of the bay as nursery habitat for species such as Dungeness crab and English sole change? What would be the effect on the livelihoods of those who fish these species? These are big issues to consider. They also remind us that there are many downstream benefits of fresh water flowing from the Delta into San Francisco Bay.