Spatial and Temporal Analysis of Chinook Salmon Redds from Historical and Current Aerial Surveys on the Cowlitz River, Washington

We investigated the spawning patterns of fall and spring Chinook salmon Oncorhynchus tshawytscha on the Cowlitz River, Washington using a unique set of fine- and coarse-scale temporal and spatial data.  Coarse-scale spatial data (500 m – 28 km resolution) were collected from 1991-2009, and fine-scale spatial data (100 – 500 m resolution) were collected in 2008 and 2009 from bi-weekly helicopter flights over the lower Cowlitz River.  We examined spatial patterns of Chinook salmon redd reoccupation among and within years in relation to segment-scale features, some of which have an impact on groundwater/surface water interactions.  Depth discontinuities, channel bifurcation, and tributary junctions are directly related to groundwater flow in the hyporheic zone.  Distance upstream, sinuosity, and channel gradient are all geomorphic factors that have been known to affect spawning distributions.  Continuous surveys were used to provide a detailed look at the landscape, along with data from a digital elevation model (DEM), topographic maps and aerial photos.  We used regression with a general linear model (GLM) to determine the relationship between habitat features and the redd locations from 2009 on a 500-m scale.  Preliminary results indicate Chinook redds were spatially clustered (P < 0.02) and occurred in the same sections each year with little variation among and within years.  Distance upstream, channel bifurcation, and sinuosity were associated with redd density on a 500-m scale.  Channel bifurcation was positively associated with the occurrence of spawning in reaches in the Cowlitz River.  These multiple channels are associated with hyporheic flow on a large scale which is critical for incubating salmon.  Increased intragravel flow often occurs at the upstream and downstream ends of channel bars or islands where the river is slower and shallower.  Determining the pattern of fall and spring Chinook salmon spawning within the Cowlitz River and its relation to habitat features is necessary for conservation and restoration.