Two Dimensional Modeling of the Extent and Stability of Cold Water Refugia at the Confluences of the South Fork of the Eel River

Abstract:

Like many large rivers draining from forested uplands, the South Fork of the Eel River is characterized by a warm main stem fed by small cold water tributaries along its length. The confluences of the tributaries with the main stem form localized zones of cold water which provide habitat for anadromous fish species such as salmon. These cold water refugia are likely to be increasingly important for fish as temperatures warm and rainfall becomes more erratic due to projected climate change. These sites are protected from drying out over summer (unlike the tributaries, which can fragment if flows become too low), and they are accessible to migrating fish (as tributaries may not be if winter flows are too low to submerge in-channel obstacles). The overall significance of these refugia, however, is controlled by their spatial extent and temporal persistence. The factors that determine the shape, persistence, and stability of these refugia (as a function of varying flow and temperature regimes) remain poorly understood. A combination of scaling arguments and a 2D model is used to estimate the effects of varying flow and thermal regimes on the spatial extent of the cold water zones in the main stem. The stability of the cold water zones is also assessed in light of diurnal flow and temperature variations. The results are compared to in situ temperature data collected at the confluence of the South Fork of the Eel River and Elder Creek, and are used to project the likely significance of tributary confluences for generating cold-water habitat throughout the South Fork of the Eel River.