- The thermal regime of rivers plays a key role in aquatic ecosystem health. In the Willamette River, OR, the present day main channel temperature is often too warm for cold water fishes during the late spring, summer, and early fall. At these times, cold water fishes tend to be concentrated in cold water refuges, legally defined as having temperatures at least 2 °C cooler than the mainstem, and typically found in secondary channel features. These include side channels, ponds, and alcoves, the last being connected to the main channel only at their downstream ends.
To determine the processes that control the thermal regime of secondary channel features on the Willamette River, we used temperature measurements from 1 side channel, 8 alcoves, and 6 beaver ponds. Monitoring stations formed a three-by-three array of thermistors at upstream, mid-, and downstream distances along each feature at near-bed, middle, and near-surface depths. Channel geometry, flow velocity, shade, and dissolved oxygen were also measured.
Field measurements and aerial photographs yielded calculations of 3 dimensionless quantities, 1) Richardson, 2) advection, and 3) hyporheic numbers. The Richardson number is the ratio of buoyancy to flow shear, describing stratification. Values below 0.25 characterize a well-mixed water column. Two of our sites, the side channel and an alcove with significant surface water flow, were well-mixed. The advection number is the ratio of a relative heating (or cooling) flux from upstream to incoming solar radiation; it is a function of depth for points within stratified features, where “relative” fluxes are calculated from differences between temperature at points in the alcove or pond and the mainstem. At points where temperatures are cooler than the mainstem (e.g. near the bed), our advection numbers were negative (-6035 < Ad < 0). Likewise, positive advection numbers (0 < Ad < 3027) described points warmer than the mainstem (e.g. near the surface).
Near-bed temperature measurements for all stratified alcoves and ponds served to ground-truth our calculated values of the hyporheic number. More predictive than descriptive, the hyporheic number is a ratio of a cooling flux from hyporheic discharge to incoming solar radiation. The cooling flux reflects estimated lengths and hydraulic conductivities for the inferred subsurface flow path as well as the mean annual mainstem temperature (11.3 °C on the Willamette). Hyporheic numbers for stratified alcoves and ponds were all positive. With the exception of 1 beaver pond, all sites with a hyporheic number ³15 had locations cooler than the mainstem. Furthermore, a plot of advection number vs. hyporheic number produces a linear fit with R2 = 0.91.
On the Willamette River, secondary channel features that provide cold water refuges are characterized by 1) stratification, and 2) cool hyporheic discharge. Stratification is a necessary yet insufficient condition for cold water. Additionally, alcoves or ponds that do meet the legal requirement for a cold water refuge may be anoxic, as suggested by dissolved oxygen point measurements at 2 of our alcoves. Even so, the predictive hyporheic number has the ability to classify the thermal regimes of secondary channel features based on aerial photographs, gage measurements, and minimal field measurements. Ultimately, these calculations have the potential to guide both floodplain preservation and restoration efforts in a more quantifiable direction.