Optimal approach for upstream fish passage design: One-size-fits-all or made-to-order?

Nearly 60,000 large dams (higher than 15 m) occur worldwide in addition to an estimated 16 million smaller impoundments with individual surface areas larger than 100 m2. The resulting habitat fragmentation threatens global riverine biodiversity and sustainable fish populations. Two opposing approaches for selecting fish passage designs to mitigate river fragmentation are possible: develop a limited number of standardized (reference) designs from which a design for a candidate dam is selected (one-size-fits-all approach) versus conduct scientific fish passage studies specific to each dam and targeted fish species (made-to-order approach). The two approaches vary in probability of effectiveness, cost of supporting biological studies, and overall project cost and schedule impact. To address this conundrum, we analyzed 73 USA dams to identify two groups that differed markedly in fish passage planning approaches. Snake River dams are similar in design, flow, geological setting, and target fish species. By contrast, Mississippi River dams are relatively dissimilar in design, flow, and geological setting but generally similar in target fish species. We conclude that the more similar a candidate dam for fish passage is to a reference set of similar dams (i.e., the Snake River dams), then the more likely fish passage technology can be successfully extrapolated to a proposed dam in the same or a nearby watershed. As a general strategy, we recommend that dams in a region be clustered using key hydrologic, structural, operational, and biological variables. These variables can be used to assign a new dam or retrofit an existing dam to a cluster to which they are most similar, thereby optimally extending existing knowledge to new applications. In the process, reliance on the less efficient and more expensive made-to-order approach can be reduced.