About Fish Passibility Estimate

The criteria used for determining passability of fish traveling upstream through road-stream crossings are a screening-level tool. It is important to note that the calculated score and passability rating given for fish passage should not be considered an absolute but rather an indication of whether the road-stream crossing structure may provide fish passage.

Passability estimates were designed to represent the proportion of individual fish of all species and life stages that could pass through the road-crossing structure going upstream. The criteria are probably conservative for salmon and trout which are strong swimmers and jumpers relative too many of our native Michigan fishes. Therefore salmon and trout may be able to pass through higher-velocity flows and jump over small outlet drops.

The criteria may be viewed below or as Appendix 1 in the protocol titled “Great Lakes Road Stream Crossing” which is located at the following link: http://www.michigan.gov/dnr/0,4570,7-153-10364_52259_55492---,00.html.

Appendix 1. Criteria for determining passability of road crossings by Michigan and Wisconsin stream fishes.

  • Passability = 0

    Most species and life stages cannot pass at most stream flows. These criteria do not apply to open bottom structures.
    • The outlet of the structure is perched, or
    • The ratio of the structure water depth to the stream water depth is less than 0.1, or
    • The water velocity in the structure is greater than 3 ft/s during baseflow.

  • Passability = 0.5

    Some species and/or life stages cannot pass at most stream flows. These criteria do not apply to open bottom structures.
    • The water depth in the structure is less than 0.2 feet, or
    • The water velocity in the structure is 2-3 ft/s during baseflow, or
    • The structure is longer than 30 ft and does not have natural substrate through its entire length.

  • Passability = 0.9

    Barrier at high flows.
    • The constriction ratio (structure width/bankfull stream width) is less than 0.5, or
    • There is a scour pool below the structure.

  • Passability = 1

    No passage problem.
    • None of the criteria above are met.