In 2021, New York assessed Cannonsville Reservoir as impaired after a 7-foot culvert blocked fish passage. 2 years later, the new culvert scored 0.88 for aquatic passage

Stream restoration projects can provide benefits that extend beyond the immediate area where construction takes place. A culvert that changes natural water flow can contribute to erosion, sediment movement, water-quality problems, and barriers to aquatic life. In Delaware County, New York, replacing an undersized culvert along an unnamed tributary to the Little Delaware River addressed these concerns while improving fish passage and reducing sediment flowing toward the Cannonsville Reservoir, a drinking-water source for New York City.

Environmental Problems Caused by the Old Culvert

The original structure was an undersized 7-foot-diameter metal culvert. It changed the stream’s hydrology, resulting in sediment to accumulate upstream and creating channel erosion downstream. The altered flow also led to the formation of downstream plunge pools that prevented native fish and other aquatic organisms from moving upstream.

The downstream erosion generated turbid water and transported sediment toward the Cannonsville Drinking Water Reservoir. Sediments entering reservoirs can carry phosphorus, which can negatively impact water quality. In 2021, New York assessed the Cannonsville Reservoir as impaired for fishing because of phosphorus inputs from point and nonpoint source (NPS) pollution.


Although the unnamed tributary faced water-quality concerns, it was not listed on the Clean Water Act (CWA) section 303(d) list of impaired waters or the Integrated Report, for unspecified causes.

New Culvert Designed to Improve Stream Conditions

To address the challenges, project partners substituted the old metal culvert with a 9-foot by 13-foot concrete box culvert. The replacement was designed to create a natural stream channel bottom, offer additional flow capacity, and minimize streambank erosion. It was also made to withstand a 100-year storm event.

Delaware County Soil and Water Conservation District (DCSWCD) staff supplied hydrology information for calculating common open channel flow hydraulic characteristics. The project also utilized culvert design methodology from the U.S. Department of Transportation’s 2012 document, Hydraulic Design of Highway Culverts. U.S. Army Corps of Engineers (USACE) guidance was used to find the appropriate rock sizes for the rock structures.

The new box culvert addressed scouring of the road above the crossing and erosion along the upstream and downstream stream embankments. It also offered improved passage for fish and other aquatic organisms.

Aquatic Passability Improved

After construction, the new box culvert received an aquatic passability score of 0.88 under the North Atlantic Aquatic Connectivity Collaborative (NAACC) aquatic passability protocol.

As per the NAACC scoring system, a score from 0.80 to 0.99 represents an “insignificant barrier” for fish and other aquatic organisms. A score of 1 represents “No barrier,” while lower scores indicate progressively greater barriers.

An NAACC score was not recorded for the old culvert before construction. However, the old culvert and its associated plunge pool were visually evident as a significant barrier to aquatic passage. The new score of 0.88 therefore illustrates improved aquatic connectivity following the replacement.

Benefits for Cannonsville Reservoir

The project also produced visible improvements in sediment movement. Following the culvert replacement, there was a visible reduction in sediments and associated nutrients flowing toward the Cannonsville Reservoir.

The improvement complements watershed-wide best management practices (BMPs) that have been used since the 2021 assessment to reduce nutrients and sedimentation, as identified in New York City’s watershed management plans. Although the tributary was not itself listed as an impaired waterbody, the restoration enhanced water quality and restored fish passage.

Partners and Project Funding

The project involved the Delaware County Soil and Water Conservation District (DCSWCD), Delaware County Department of Public Works (DCDPW), Keystone Associates, U.S. Army Corps of Engineers (USACE), New York City Department of Environmental Protection, Town of Bovina, and New York State Department of Environmental Conservation (NYSDEC).

The total construction cost was $344,896, including $182,586 in NYSDEC Water Quality Improvement Project (WQIP) funds. DCSWCD provided $162,310 in local cost-share funding through its contract and partnership with the New York City Department of Environmental Protection. DCDPW completed the topographic survey, while federal CWA section 319 Performance Partnership Grant funding supported salaries of state staff involved in awarding WQIP grants.

Overall, the culvert replacement improved stream conditions, strengthened aquatic passage, reduced erosion, and visibly reduced sediment reaching the Cannonsville Reservoir.

Source: PDF from the U.S. Environmental Protection Agency (EPA)

FAQs:

Q1. What is a culvert?

A culvert is a structure that allows water to pass beneath a road or other crossing. Its design can affect stream flow and aquatic connectivity.

Q2. Why are culverts important?

Culverts help manage water movement beneath roads and crossings. Properly designed structures can also support fish and aquatic organisms.