HOW HAS CLIMATE CHANGE ALTERED NETWORK CONNECTIVITY IN A MOUNTAIN STREAM NETWORK?

Connectivity along river networks is broadly recognized as dynamic, with seasonal and event-based expansion and contraction of the network extent. Intermittently flowing streams are particularly important as they define a crucial threshold for continuously connected waters that enable migration by aquatic species. In the Pacific Northwestern U.S., changes in atmospheric circulation have altered rainfall patterns and decreased summer low-flows in the region. However, the impact of these changes on network connectivity is heretofore unstudied. Thus, we ask: How has connectivity in the riparian corridor changed in response to observed changes in climate?

We use the H.J. Andrews Experimental Forest as a representative mountain river network from the Pacific Northwest . First, we analyze 63 years of stream gauge information from a network of 11 gauges to document observed changes in timing and magnitude of stream discharge. We found declining magnitudes of seasonal low-flows and shifting seasonality of water export from the catchment, both of which we attribute to changes in precipitation timing and storage as snow vs. rainfall.

Next, we use these discharge data to drive a reduced-complexity model of the river network to simulate network connectivity over 63 years. Model results show that the extent of the stream network, at summer low flow, has been decreasing. Unexpectedly, the increasing winter peak flows did not correspond with increasing network expansion, suggesting a geologic control on maximum flowing network extent. We find dynamic expansion and contraction of the network primarily occurs during period of catchment discharge less than about 1 m³/s at the outlet, whereas the network extent is generally constant for discharges from 1 to 300 m³/s.

Results from our study will help understand the role of connectivity as a control on ecological processes both directly (e.g., fish migration) and indirectly (e.g., stream temperature modeling). Additionally, our results inform management and regulatory needs such as estimating connectivity for entire river networks as a basis for regulation, and identifying the complexity of a shifting baseline in identifying a regulatory basis.