Nature-Like Fishways: Proceed, but Use Caution

The fundamental reason why nature-like fishways hold such promise is that the closer a fishway resembles natural river conditions, the more native species and life stages it will pass. For this reason, removing dams is the preferred option for long-term, self-sustaining fish passage. In places where removal is not a viable option, nature-like fishways, such as rock ramps and bypass channels, can more closely simulate river conditions and therefore should be considered as the next best option where feasible. However, any technical or structural fish passage, including nature-like fishways, will either fail at some point under pressure of fluctuating water or have their effectiveness compromised, especially structures that need to maintain specific water levels and attraction flows. An effective design can produce a long structure life, but that life will be finite. An increasing number of studies from around the country are showing high failure rates of instream structures. Alternatives analyses frequently overlook this and consider nature-like fishways to be walk-away solutions. There are several failure mechanisms for structures in rivers including: design flows are exceeded; over-dependence on hydraulic design techniques that are developed for less turbulent flow; long-term undermining of channel-spanning structures due to scour; flanking of structures at high flows; or outright poor design. Having worked on more than one hundred fish passage projects around the country, American Rivers will present examples of failures of instream structures, which provide valuable information for fish passage designers. We have developed several recommendations for nature-like fishways, including: 1) Pursue dam removal – more than 885 dams have been removed around the country, and the option is frequently viable if pursued aggressively. Any technical fishway should be used only as a last resort on dams that are in good condition, will be maintained, and serve important purposes. 2) Have appropriate expectations for long-term viability of any structure in a river. 3) Develop a long-term operations and maintenance plans for all technical fishways. The degree of caution can be conceptualized by the stream power of the site. Failure risk is higher where the discharge is high or the slope is steep. 4) Simulate the surrounding river to the extent possible. 5) Design for multiple flows, not just passage flows for target species: consider structural integrity at high flows; design for low flows to avoid stranding resident species. Overall, recognize that any structure can fail in flowing water and proceed with appropriate caution.