Urban Stream Temperatures Under Warm Climate Regimes

The recent high-intensity California drought and the very recent undoubtably warm years may affect urban streams in different ways than those less influenced by human activity. Urban streams in highly seasonal, water limited environments, such as those in the southern San Francisco Bay Area, provide the southernmost habitat for chinook salmon and steelhead trout. These streams also provide other important ecosystem services for water supplies and storm water drainage close to urban population centers. Urban stream systems may also be particularly vulnerable to impacts from increased air temperatures and drought, as stream function has been impacted by channel modifications, destruction of riparian zones, and pollution. Yet, the effects of climatic variability and change on urban streams remain understudied. Here we compare the stream temperature variability and change between urban and non-urban settings over several climatic regimes, including a period of extreme drought in northern California (2011 - 2016). Hourly stream temperature records from a network of 18 urban stream sites in the urban regions of the southern San Francisco Bay Area represented a variety of stream environments and were compared to non-urban sites and historical records using statistical and GIS analysis. Our results showed that the frequency of occurrence of periods of elevated stream temperatures are greater during both warm and dry and warm and wet periods for both urban and non-urban streams, but urban streams remain warmer through all climatic conditions. Reservoirs and removal of riparian shading further increases stream temperatures in urban streams, crossing temperature threshold requirements for cold water fish. Managing stream temperature under warmer climates may be particularly important for urban streams.