However, there are times when crossing a stream cannot be avoided and a structure that will fit the site and accommodate the required traffic must be designed and installed. Selecting the appropriate structure is a critical step towards insuring that a crossing is both usable and cost effective, and that pollution of the stream is minimized. The most basic stream crossing structure is the ford . If you are fortunate enough to have a gravelly or sandy stream bottom with good bearing strength, the natural ford requires the least amount of improvement to allow vehicles to cross and is therefore the least costly approach initially. If the stream bottom is of less stable material such as a silty or clayey soil, a ford can be constructed of rock and man-made retaining materials.
Installation costs for a man-made ford can approach several thousand dollars per crossing.
The most common type of stream crossing structure is the culvert crossing . The culvert should also be stabilized with rock, grass or other soil stabilizing material to minimize erosion and sediment movement into the stream. Finally, the culvert crossing requires frequent maintenance to insure that debris does not clog the culvert inlet and reduce the water flow through the culvert, which may result in additional erosion around the culvert. In extreme cases, the culvert may be washed out if not properly designed or maintained. Generally, the most costly type of stream crossing structure is the bridge .