With El Niño making an impressive new year’s debut, talk of drought has turned to worries about floods. How will the state’s drought-starved ecosystems adapt to the taps being turned on again? We talked with Joshua Viers, an ecological engineer at UC Merced and a member of the PPIC Water Policy Center research network, about California ecosystems’ response to flooding.
PPIC: Are floods "natural disasters” for nature?
Josh Viers: Disturbances like floods, droughts, and fire are regular features in California, and our ecosystems are fairly well adapted to these extreme events. There has been a lot of evolutionary adaptation to these disturbances over time, and this is one reason California has such high biodiversity. What compromises our ecosystems’ ability to be resilient to extreme events is that many habitats and species populations have been degraded by human activities. That makes it harder to gauge an ecosystem’s ability to snap back after extreme drought or floods.
Flooding can be quite good for aquatic ecosystems, especially after years of drought, because it reorganizes the physical habitat by moving rocks, logs, and sediments. If water overflows into the floodplain, it can recharge shallow groundwater, replenish soil nutrients, distribute seeds from native plants, and create new habitat for animals and plants.
PPIC: How does California’s flood-control infrastructure affect the ability to manage water for the environment?
JV: California has one of the most elaborate water infrastructure systems in the world. Over the decades, it has had both positive and negative effects on our ecosystems. Our system of dams and canals has degraded habitat, and severely altered the natural variability of river flows, which can disrupt ecosystem functioning and species’ life cycles. But we’re increasingly able to manage the system to mimic rivers’ natural flows and improve long-term ecosystem functioning. As the climate warms, however, we will need to manage water flows to ensure downstream water temperatures don’t get too high for many of our native fish species.
PPIC: What changes to our flood management system could improve environmental conditions?
JV: We need better forecasting on the amount of water coming into reservoirs, so we can minimize the impacts of sudden high releases from dams, which can damage ecosystems downstream. Being able to "pre-wet” ecosystems dried from the drought can help reduce the effects of a sharp change. For example, in fall the Cosumnes River had many native fish waiting to swim upstream to spawn, but its dry channel forced them to wait longer than normal. The first rains just saturate the streambed. Pre-wetting it allows these rains to become streamflow and help get those fish moving upstream on time.
A longer term fix would be to set back more levees, which would give rivers room to flood. This can have ecosystem benefits and reduce property damage from floods. We’ve identified a number of aquatic ecosystem benefits from this approach. The wider channel slows floodwaters and supports growth of phytoplankton, which are the base of the food chain for a whole host of creatures. Juvenile salmon grow much bigger and more quickly in this environment. That means they can survive their ocean migration much better and are more likely to come back to spawn. Additional benefits include forest regeneration, nutrient and sediment deposition (which creates new habitat for riparian plants), and local groundwater recharge. These whole-ecosystem responses have human benefits as well.
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