HYPORHEIC AND GEOMORPHIC CHARACTERISTICS OF SPRING CHINOOK SALMON SPAWNING HABITAT OF THE YAKIMA RIVER BASIN, CENTRAL WASHINGTON

In order to aid salmon restoration efforts in the Pacific Northwest, it is essential to understand the physical conditions required for spawning. Certain characteristics of the hyporheic zone, including vertical head gradient, permeability, and nutrient flux, provide cues for salmon in the selection of spawning sites. Fluvial geomorphic processes control many of the physical conditions that are required for spawning habitat, including the nature of interstitial flow pathways between surface water and groundwater. In this study, comparisons were made of hyporheic and geomorphic variables between spawning reaches and non-spawning reaches on two Yakima River tributaries: the American River and Little Naches River. Vertical head gradient, permeability, and nutrient levels were measured from piezometers installed in transects across each reach. Geomorphic parameters, including channel dimensions, sinuosity, longitudinal gradient, and side channel abundance, were compiled for each reach using existing data. The objective of this study is to develop a conceptual model describing the physical features of high-density spring chinook spawning habitat and thus assist decision-makers in evaluating future management strategies.

Groundwater influx through the hyporheic zone, characterized by higher electrical conductivity of hyporheic waters and high substrate permeability, was detected in spawning reaches in both rivers and is considered to be an important feature of suitable spawning habitat. In addition, substrate porosity was greater within both spawning reaches at a statistically significant level. Non-spawning reaches tended to have strongly downwelling vertical head gradients, while these gradients were weakly downwelling to upwelling within spawning reaches. Geomorphically, salmon spawning reaches were characterized by relatively wide, low-gradient, sinuous channels with high connectivity to the floodplain and a high abundance of side channel habitat. These geomorphic characteristics promote connectivity between the river and groundwater and give rise to local hyporheic conditions conducive to successful spawning. Based on these results, we conclude that salmon select spawning areas within reaches that are geomorphically complex and have a high degree of hyporheic exchange.