Evaluating the effects of climate change on managed lowland rivers in the California Bay Delta Watershed

Water that is released from reservoirs can affect the downstream thermal regimes of rivers. During the summer months, these flow releases can lower the river temperature downstream of dams in an extension that mainly depends on the volume and temperature of these releases and the energy exchange with the overlying atmosphere. The benefit of this cooling effect has been suggested as an approach to mitigate the effects of climate change in downstream-regulated rivers. However, anticipated climate change conditions may weaken these cooling benefits, especially in managed lowland rivers (MLRs), as they are subjected to large withdrawals, are shallow, and convey clear water. Here, we show that MLRs in the California Bay Delta Watershed are vulnerable to water temperature increases, especially during future summer months subjected to a future high-emission greenhouse scenario. Low-flow conditions exacerbate this vulnerability, especially at locations downstream of high-flow diversions. By using a physical energy balance model (FLUVIAL-EB) paired with a downscaled climate regional model (CRCM5-RCP8.5), we found that for summer months between 2030 and 2100, longwave and latent heat fluxes will contribute to water temperature increases, while absorbed solar radiation will likely decrease under future climate scenarios. Despite the warming effects of climate change on MLRs in the California Bay Delta Watershed, our findings suggest that increasing the release of hypolimnetic water from reservoirs during summer months can be a viable solution to mitigate the river temperature increase.