Food Production as Estimated by Drift in a Managed Watershed During Summer Baseflow

The Calapooia River watershed in the Willamette Valley of Oregon drains a forested area managed for timber for more than a hundred years.  The watershed supports two anadromous salmonids, Winter Steelhead (Oncorhynchus mykiss) and Spring Chinook (Oncorhynchus tshawytscha); both are listed as threatened under ESA.  The early loggers harvested the riparian forest and transported the logs using the river, causing a lack of large wood, elevated water temperatures, and impaired hyporheic exchange. 

We examined species viability in the watershed from an energetic perspective by measuring food production, stream temperature, and rearing habitat availability in tributary and mainstem locations during baseflow.  Summer baseflow has been shown to be an energetic bottleneck for salmon and steelhead young.  Drift was measured in fifteen tributaries ranging in size from 1 to 3^rd order (2004) and eight randomly chosen sites in the mainstem (2010) weekly in August.  Flow adjusted drift rates were similar at mainstem and tributaries sites ranging from <1 to 13 invertebrates/m³.  At all sites drift rate declined with declining discharge.

Rearing habitat is rare and stream temperature maxima approach species thermal limits at lower mainstem locations with maxima > 5⁰ C cooler in the upper mainstem.  Initial results show food is not limiting, although thermal maxima necessitate a large ration to support growth.  These factors limit abundance and constrict the distribution of suitable habitat for both Spring Chinook and Winter Steelhead.  Broadly applied measures to increase roughness for habitat expansion and development of hyporheic exchange in conjunction with growth of riparian forests appear needed for the system to sustain anadromous salmonids.