Developing a grassed waterway

by Sally Colby
Soil and water are among any farm’s most valuable resources, but the two don’t always make the ideal combination. Water in the form of runoff is a powerful force, and can remove significant amounts of soil from agricultural land. To solve the problem of runoff and to protect soil, water can be effectively diverted to a desirable location by means of grassed waterways.
The NRCS (Natural Resources Conservation Service) defines a grassed waterway as ‘a shaped or graded channel that is established with suitable vegetation to carry surface water at a non-erosive velocity to a stable outlet’. The purpose of the waterway is to divert water without causing erosion, to reduce or eliminate gully erosion and to protect and improve water quality. Although the construction of a grassed waterway will likely result in the loss of some potentially productive cropland, the overall benefits outweigh the loss.
The EPA requires that the minimum capacity of a grassed waterway should adequately handle the peak runoff expected from a 10-year frequency 24-hour storm. The capacity should be increased as necessary to allow for the potential volume of sediment that may accumulate in the waterway between planned maintenance activities.
The farmer who determines that a grassed waterway is the answer to erosion has several steps to achieve the end result. Because grassed waterways must comply with federal, state and local laws and regulations, begin by contacting a soil conservationist/engineer through NRCS. Although the farmer will have an idea of where the waterway is needed, the engineer will be able to determine the overall size of the waterway, the slope of the watercourse, and the drainage area. Although the finished waterway looks relatively simple, each waterway is unique and requires individual attention to its design. Ideally, waterways are constructed when the surrounding land is not being used for crop production.
After site exploration, viewing farm maps, examining crop history and looking at GIS data, the NRCS engineer will develop a plan for the waterway, taking into account a number of factors including construction that will closely follow the field’s natural drainage path. The engineer also considers the level of erosion that can potentially occur during construction of the grassed waterway, and may recommend the use of biodegradable erosion control blanket during establishment. Additional structures such as straw bales, fabric checks, filter fences or mulch might be necessary to protect the waterway until the permanent vegetation is established.
Newly constructed waterways are seeded immediately after the seedbed is prepared. The NRCS engineer will recommend appropriate species for the waterway, which are selected based on factors including soil type, slope, water carrying capacity and suitability for the region. Species will include a combination of fast-growing annuals and hardy perennials. Some waterways are suitable for flowering species (such as clovers) that support native pollinators, and many landowners include such species whenever possible.
After the waterway is complete, inspect the newly-seeded waterway for bare or eroded areas frequently and make appropriate repairs. Heavy rain can disturb the immature grass planting, so landowners should check waterways for signs of damage. Apply fertilizer according to NRCS recommendations, and make the application when the risk of runoff is low. Mow the waterway several times during the growing season and consult NRCS for advice in controlling noxious weeds.
Once mature, the grassed waterway should be maintained as a part of an ongoing whole-farm soil health effort. When farming the acreage surrounding the grassed waterway, be careful not to damage the sod with implements or sprays. Avoid the temptation to use the waterway as a turn row while performing field operations.
Farmers who see signs of erosion during the upcoming growing season can take photographs throughout the season, especially during and after heavy rain, to document the changes in that field. Although photographs aren’t necessary, they can aid the engineer in the planning process.