Effect of river confinement on depth and spatial extent of bed disturbance affecting salmon redds

ABSTRACT

Human impacts on rivers threaten the natural function of riverine ecosystems. This paper assesses how channel confinement affects the scour depth and spatial extent of bed disturbance and discusses the implications of these results for salmon-redd disturbance in gravel-bedded rivers. Two-dimensional hydrodynamic models of relatively confined and unconfined reaches of the Cedar River in Washington State, USA, were constructed with surveyed bathymetry and available airborne lidar data then calibrated and verified with field observations of water-surface elevation and streamflow velocity. Simulations showed greater water depths and velocities in the confined reach and greater areas of low-velocity inundation in the unconfined reach at high flows. Data on previously published scour depth of bed disturbance during high flows were compared to simulated bed shear stress to construct a probabilistic logistic-regression model of bed disturbance, which was applied to spatial patterns of simulated bed shear stress to quantify the extent of likely bed disturbance to the burial depth of sockeye and Chinook salmon redds. The disturbance depth was not observed to differ between confined and unconfined reaches; however, results indicated the spatial extent of disturbance to a given depth in the confined reach was roughly twice as large as in the unconfined reach.

KEYWORDS:

• Confined channel
• scour
• salmon redd
• two-dimensional hydrodynamic modelling
• human impacts
• streambed disturbance

Acknowledgments

The authors would like to thank Rand Little and Karl Burton of Seattle Public Utilities, as well as the other members of the Cedar River Instream Flow Commission, for their valuable insights and fish-spawning data on the Cedar River and help developing the themes of this paper, King County Department of Natural Resources and Parks for access to lidar and orthoimagery data, and Jon Nelson and Rich McDonald of the U.S. Geological Survey for their instruction and support with the iRIC modelling system. We thank Rich McDonald and anonymous reviewers for their insightful comments. ASM data are available from Gendaszek et al. (2013). Streamflow gaging data from the Cedar River are from U.S. Geological Survey (2012). Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government, the authors or their affiliations.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was funded by Seattle Public Utilities; U.S. Geological Survey under the Cooperative Water Program.