How Flood Disturbance Structures The Spatial Pattern Of Mussel Beds and Salmon Spawning Redds In A Large River

Predicting the spatial distribution of sensitive species, and their risk from flood disturbance, is a significant challenge for conservation.  Hydrodynamic modeling, coupled with field observations, can provide important insight into spatial patterns of benthic disturbance during floods by predicting stresses on the streambed.  Our study is located on the Trinity River in northern California and focuses on two imperiled populations of western pearlshell mussel (Margaritifera sp.) and chinook salmon (Oncorhynchus tshawytscha).  In order to determine the risk of disturbance to both species during floods, we quantified the relation between river discharge, bed mobility, and scour and fill depth.  Our results reveal that mussel beds are preferentially located in areas of the streambed that are least likely to become mobilized or risk deep scour during floods, and their occurrence can be predicted from simple parameters (water depth, velocity at baseflow, grain size, and distance to the streambank).  Similarly, salmon redds are also located in areas of the streambed with a low risk of scour, but do not co-occur with mussel beds due to slight differences in habitat use.  These results indicate that both species have evolved successful strategies for surviving and reproducing in flood-prone ecosystems, and that their spatial distribution can be predicted from patterns of flood disturbance.