Aquatic Organism Passage: A Princeton Hydro Blog Series

Introducing part one of a multi-part blog series about aquatic organism passage
What you’ll learn:
  • What is aquatic organism passage?
  • Why is it important?
  • How does Princeton Hydro support it?

This photo from NYS DEC demonstrates a well-designed stream crossing.

Since the US government began allotting funds for building roads in U.S. national forests in the late 1920s, hundreds of thousands of culverts were built across the country. Culverts, or drainage structures that convey water underneath a barrier such as a road or railroad, were originally built with the intention of moving water quickly and efficiently. While this goal was met, many migratory fish and other aquatic organisms could not overcome the culverts’ high-velocity flows, sending them away from their migratory destinations. If the culvert was perched, or elevated above the water surface, it would require the migratory aquatic animals to both leap upwards and fight the unnaturally fast stream current to continue their journeys. Additionally, turbulence, low flows, and debris challenged the movement of aquatic organisms.

Thus, the goal of aquatic organism passage (AOP) is to maintain connectivity by allowing aquatic organisms to migrate upstream or downstream under roads. AOP “has a profound influence on the movement, distribution and abundance of populations of aquatic species in rivers and streams”. These aforementioned species include “fish, aquatic reptiles and amphibians, and the insects that live in the stream bed and are the food source for fish”.

This photo from NYS DEC demonstrates a poorly-designed stream crossing.

A poorly designed culvert can harm fish populations in multiple ways. If sturgeon aren’t able to surpass it, habitat fragmentation prevails. And so, a once-connected habitat for thousands of sturgeon breaks into isolated areas where a few hundred now live. When the population was in the thousands, a disease that wiped out 80% of the population would still leave a viable number of individuals left to survive and mate; a population of a few hundred will be severely hurt by such an event. In sum, habitat fragmentation raises the risk of local extinction (extirpation) as well as extinction in general.

The splintering of a large population into several smaller ones can also leave species more vulnerable to invasive species. Generally, the greater the biodiversity harbored in a population, the stronger its response will be against a disturbance. A dwindling community of a few hundred herring will likely succumb to an invasive who preys on it while a larger, more robust community of a few thousand herring has a greater chance of containing some individuals who can outcompete the invasive.

Aquatic Organism Passage in Action at Princeton Hydro

Princeton Hydro recently teamed up with Trout Unlimited to reconnect streams within a prized central-Pennsylvanian trout fishery.  Our team enabled aquatic organism passage by replacing two culverts in Pennsylvania’s Cross Fork Creek. Read about it here!


“Aquatic Organism Passage through Bridges and Culverts.” Flow. Vermont Department of Environmental Conservation’s Watershed Management Division, 31 Jan. 2014. Web. 14 Mar. 2017.

Hoffman, R.L., Dunham, J.B., and Hansen, B.P., eds., 2012, Aquatic organism passage at road-stream crossings— Synthesis and guidelines for effectiveness monitoring: US Geological Survey Open-File Report 2012-1090, 64p.

Jackson, S., 2003. “Design and Construction of Aquatic Organism Passage at Road-Stream Crossings: Ecological Considerations in the Design of River and Stream Crossings.” 20-29 International Conference of Ecology and Transportation, Lake Placid, New York.

Kilgore, Roger T., Bergendahl, Bart S., and Hotchkiss, Rollin H. Publication No. FHWAHIF-11-008 HEC-26. Culvert Design for Aquatic Organism Passage Hydraulic Engineering Circular Number 26. October 2010.

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