Skip to main content

U.S. Forest Service

Aquatic Organism Passage Program Glossary

Aquatic Organism Passage (AOP) - AOP is the ability of fish and other aquatic organisms to migrate and swim freely upstream and downstream through or beneath human infrastructure such as culverts, bridges, diversion, dams, etc. Assessments from the past decade confirm the ecological importance of providing passage to all native aquatic and riparian species dependent on passage through road-stream crossings and for all life stages of those species, particularly to provide resilience to climate change stresses to these populations.

Bankfull - Describes the volume of flow, and the flow width or depth associated with the bankfull elevation: that point where water fills the channel just before beginning to spill onto the flood plain.

CMLG - The internal Forest Service funding code for Legacy Roads and Trails money.

CMRD - The internal Forest Service funding code for Capital Improvements and Maintenance work that includes road maintenance and decommissioning.

FishXing - This is a software program developed by the U.S. Forest Service, U.S. Fish & Wildlife Service, state, and other partners to design road-stream crossings and fish ladders to enable certain species of fish and life stages of fish to pass under or through human infrastructure such as a culvert, bridge or dam. See Aquatic Organism Passage: FishXing.

Road-Stream Crossing - A road-stream crossing is the interface of the road network and a stream, creek or river, typically containing the water body in a culvert, pipe, arch, bridge or causeway.

Safe Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) - This Act authorizes Federal Highways Administration to fund U.S. Forest Service aquatic organism passage at $10 million per year.

Stream Simulation - Stream Simulation is a design process that is structured to be consistent with the intent of the Clean Water Act and Endangered Species Act. It is a design method that tries to make the structure as similar to the surrounding environment as possible. The structure cannot restrict the stream, velocities cannot be increased, stream-like substrate needs to be in the structure, etc. Stream Simulation requires much more analysis of the stream and can be thought of as an analysis based design process compared to hydraulic capacity design that is a standards-based design process. For more information, see:

  • FishXing - the Forest Service Stream Simulation Design approach to ensuring Aquatic Organism Passage at road-stream crossings.
  • Forest Service Stream Technology Can Prevent Road and Bridge Washouts - a USDA blog on about the strong performance of Stream Simulation Design road-stream crossings on the Green Mountain National Forest during the devastating 2011 flood from Tropical Storm Irene in Vermont

Undersized Culvert - Many culverts, or pipes, that are used to carry water beneath a road, are sized well below the bankfull width of the stream and cause a variety of biological, hydrologic and geomorphic impacts. These “undersized” culverts impede water upstream of the culvert during high flow events, increasing water velocity inside the pipe, and subsequently scour the stream bottom below the pipe outlet. This decrease in water velocity upstream of the pipe causes gravel and cobble to be deposited in the stream channel upstream of the pipe, and the increase in water velocity and scour forms a deep pool and causes bank erosion downstream of the pipe, resulting in the pipe outlet over time becoming “perched” or raised above the water surface. Undersized culverts typically impede fish passage upstream through a combination of perched height above the water surface (forcing fish to jump up into the culvert), water velocity within the pipe, and lack of resting areas for swimming fish within the pipe. Undersized culverts are also more prone to catastrophic failure due to water overtopping and eroding the road approach or roadbed above and around the culvert, and due to the trapping of wood and other flood debris on the upstream side of the culvert inlet.