Recent warming of terrestrial climates around the globe has motivated concern about consequent alteration of flow, temperature and nutrients of streams ecosystems. However, since 1950 Pacific Northwest streams have also experienced other anthropogenic influences such as forest harvest, water withdrawals for irrigation and water regulation for flood control, hydropower, and recreation. It is not well understood how these collective influences might affect aquatic ecosystems, especially coldwater fishes. Using long-term historical records (1950-2010), we examined trends in (1) climate, (2) streamflow, (3) stream temperature and, (4) nutrients, for sites with and without water regulation throughout the Pacific Northwest.
Results/Conclusions
Streamflow in headwaters (unregulated sites) did not change and annual snowmelt runoff peaks shifted only a few days earlier over the period from 1950 to 2010. However, in the lower reaches below dams, streamflow change was dominated by very strong signals associated with reservoir operation for flood control (reduced annual peak flow) and irrigation (augmented late summer low flow), as well as flow management for hydropower, navigation, and recreation. Trends in stream temperature were mixed, with cooling and warming trends as well as reduced variation of their magnitude. Over longer periods of time, stream temperature trends were predominantly warming trends. However, during the most recent two decades (1987-2009), cooling trends dominated in both unregulated and regulated sites. Similar to stream temperature, trends in stream nitrogen concentrations were mixed, with both increases and decreases, and in some cases the direction of the trend shifted depending on the length of the period of record analyzed. In particular, neighboring catchments did not necessarily show similar trends, suggesting that local perturbations may be more influential than regional factors in determining those trends. Collectively, our findings suggest that the recent warming climate appears to be not associated with simple responses of streamflow, water temperature or nutrient concentrations. Potential climate change-associated influences on stream flow and temperature trends can be easily overwhelmed by local-scale anthropogenic factors. Since 1950, aquatic ecosystems in the Pacific Northwest have been exposed to multiple climatic and non-climatic forces that vary by time period and position within the stream network, and appear to be driven by forest harvest and regeneration, water regulation, and local catchment particularities (i.e., soil composition, geology, biogeochemical cycling, etc.), as much, or more, than the recent warming climate.