DEVELOPING METRICS TO CHARACTERIZE HEAT BUDGETS IN OFF-CHANNEL HABITATS, WILLAMETTE RIVER, OR

In the Willamette River, the present day main channel exceeds regulatory temperature thresholds during much of the late spring, summer, and early fall. During these times, cold water tends to be concentrated in geomorphic features off the main channel. Alcoves and side channels offer thermal refuges to steelhead and salmon during periods of migration and spawning.

The prediction of water temperature in different locations along the Willamette can be accomplished using a heat budget framework. Primary contributions to the heat budget of a gravel bed river include, (1) advective heat transfer in the well-mixed main channel, (2) inputs from incoming solar radiation, and (3) hyporehic discharge. While not a comprehensive list, these terms may be representative enough of the heat budget to define a hierarchy of control over daily maximum temperatures in the diverse set of water features composing the Willamette and its floodplain. In a typical channel segment (e.g., ~ 1 km), advective inputs far exceed all other contributions to the heat budget. After advective and dispersive components, insolation is dominant. Contributions to the heat budget by hyporheic exchange are much smaller than the first two tiers of the hierarchy, and are therefore important in producing cooler water temperatures when those other terms are unusually small (i.e. well shaded alcoves cut off from surface flow).

This study tests the use of dimensionless numbers representing each of the three tiers in the hypothesized hierarchy to predict the thermal regime of channel and alcove segments, particularly off-channel habitats. Simple physical measurements from aerial photographs and past temperature and fish survey data allowed for the selection of distinct types of off-channel habitats. At each site, temperature measurements are used to relate the heat budget metrics to the site’s temperature profile. Ultimately, we aim to use the three-tiered hierarchy to place channel and alcove segments in gross boxes defined by advection dominance, insolation dominance, and hyporheic dominance, respectively. Furthermore, we seek to relate readily observable features, like vegetation and channel dimensions, to the relative potential of hyporheic exchange to produce cool temperatures when the mainstem temperature is high.