Designing Fishways Based on the Natural Morphology of Steep Streams

Numerous types and configurations of nature-like fishways, constructed primarily of rock and other natural materials, have been employed throughout North America, Europe, and Australia.  Design of nature-like fishways focuses on producing hydraulic conditions that satisfy passage criteria for target fish species, but seldom incorporate the stable morphology of natural channels into their design. 

In recent years, designers have employed new geomorphically-based approach to engineer and construct fishways, founded on mimicking the bed morphology and composition of steeper natural channels.  These geomorphically-based fishways are presumed to more likely satisfy broad fish passage objectives if constructed with a morphology and slope similar to natural channels that the target fish species and life-stages are known to reside and traverse.  Additionally, natural bedforms found in steeper channels are more persistent due in part to their ability to dissipate energy efficiently over a wide range of streamflows.

These geomorphically-based fishways have been applied to fish passage at small dams and road-stream crossings on smaller waterways. They are typically channel-spanning and receive the entire streamflow.  The authors have described an approach to designing these types of geomorphically-based fishways in two recent publications (CDFG, 2009; USFS, in press).  The approach is founded on using the Montgomery and Buffington (1997) channel classification system, which focuses on relationships between channel bedform and channel slope. 

A geomorphically-based fishway has a steeper slope and different morphology than the adjacent channel, and cannot adjust similarly.  The design approach includes procedures to develop a stable streambed gradation and distribution, consisting of larger rock that forms a stable framework and smaller materials to control porosity. As a result, sizing of stable bed material is a critical element in the design of these fishways.  Hydraulic resistance created by the bed morphology and composition is estimated using empirical equations dependent on water depth and streambed gradation. 

This presentation will outline an approach to design of geomorphically-based roughened channels and demonstrate through case-histories construction techniques and fishway performance.  It will also highlight the uncertainties that remain and monitoring needed to validate and refine this approach.

CDFG. 2009. Fish Passage Design and Implementation.  Part XI in Salmonid Stream Habitat Restoration Manual 3rd edition. California Department of Fish and Game.

Montgomery, D. R., and J. M. Buffington. 1997. Channel-reach morphology in mountain drainage basins. Geological Society of America Bulletin 109(5):596-611.

USFS. in press.  Design of Culvert Retrofits for Fish Passage.  San Dimas Technology and Development Center, US Forest Service.