Trend Analysis of Salmon Rearing Habitat Restoration in the Trinity River at Summer Base Streamflow, 2005-2015

A goal of the Trinity River Restoration Program is to enhance the production of naturally spawned salmonids by implementing a suite of restoration actions including streamflow management, gravel augmentation and mechanical channel rehabilitation. Short-term monitoring of select channel rehabilitation sites has documented a direct increase in rearing habitat as a result of channel construction activity; however, a companion study failed to detect substantial improvements between 2009 and 2013 at a 64-km restoration reach scale. Here, we analyzed longer term performance of channel rehabilitation sites and the effect of spatiotemporal changes to constructed and natural off channel features to inform the adaptive management process. We assessed the effect of construction, from 2005-2015, at 13 rehabilitation sites surveyed before and after construction. We also developed a sub-sampling protocol to assess trends in the amount of rearing habitat at a total of 22 rehabilitation sites. All data assessed in this report were collected at a Lewiston dam release of 12.7 cms and all analyses were applicable to that streamflow. Rearing habitat increased at 12 of 13 sites after construction. One site, Trinity House Gulch, experienced a 23% decrease in optimal presmolt habitat attributable to fluvial processes that occurred before the first post-construction survey. However, the trend analysis indicated that the level of initial benefit from construction was not sustained over longer time periods at many sites. Ten of 19 sites had less total habitat at the most recent survey than they did at the first survey after construction; 1 of those 10, Hocker Flat, had slightly more optimal habitat. The year of construction does not appear to affect the amount of habitat after construction (n=11 sites) or at the most recent survey (n=19 sites). However, six of seven sites had more habitat at the most recent survey than they did at pre-construction.