Natural watershed storage greatly influences seasonal streamflow dynamics as well as hydrological responses to projected changes in the climate. Relative to other systems in the Pacific Northwest, the rain-dominated, coastal watersheds of western Oregon have limited water storage. In general, these watersheds store little to no water as snow, the aquifers are relatively shallow and the topography is largely characterized by steep, narrow and short mountain valleys. As a result, precipitation is routed quickly through the hydrological system, causing runoff to closely follow rainfall. In the summer, stream water originates entirely from water stored in the ground, resulting in low stream flows. Under climate change, changes in the seasonal patterns of precipitation have the potential to magnify the risk of extreme streamflow conditions, including extended summer low flows and shorter, more intense winter runoff events. Changes in streamflow timing and quantity is particularly relevant to spawning Pacific salmon, who rely on specific aquatic conditions during both dry and wet seasons for juvenile survival and adult spawning.
Variables that drive watershed storage, broadly termed “hydrogeomorphic setting,” vary to some degree across Oregon Coastal watersheds, suggesting that the storage-runoff patterns are also variable. Existing research has demonstrated that seasonal patterns of streamflow are responding inconsistently to changes in precipitation across coastal systems. However, there is currently little research about the relationship between seasonal streamflow patterns and hydrogeomorphic settings in rain-dominated, storage-limited landscapes. To fill this research need, this research project will estimate the relationship between predominant streamflow source (old or new water) and hydrogeomorphic setting across Oregon Coast Range watersheds. Streams with greater proportions of old water are assumed to store more water and are therefore expected to be more resilient to changes in the climate. Findings from this research will support water resource and freshwater habitat management decisions by identifying when and where coastal streams are most vulnerable to changes in precipitation.