Erosion

 

The ultimate fate of every pond is to slowly fill and someday become a meadow. In addition to the progressive accumulation of organic matter known as eutrophy, particles of soil washed from the surrounding land descend to the pond where, in the quiet or slow moving water, some will settle. The pond will become shallower until it fills. The process which silts the pond is erosion which may be as close as the pond edge, or miles away in the watershed.

Bank Erosion- Lapping waves may gradually cut into the pond edge at the waterline. Exposed soils are pried loose by the constant agitation and carried out into the pond. Steep banks my form as the shoreline moves outward, and once formed the process accelerates as ledges are under cut, and fall into the pond. Nature’s way of protecting the shoreline are "marginal plants" which have adapted to hold soils at the water’s edge with netlike root systems, and buffer wave action with flexible stems. Manmade structures (rip rap, sheet piling, cobbles, gabion netting, and erosion walls) are engineered barriers used to address wave erosion.

A second cause for erosion on the pond banks are human or livestock activity which strip away the plant cover. These may include boat launching areas, swimming areas, or any heavily used access point.

Watershed Erosion- Tilled agricultural fields, road construction projects, mining, or any disturbance which removes protective vegetation or otherwise exposes soils to washing from surrounding lands will accelerate erosion. Soil particles are then washed into solution by rainfall, and in drainage ditches, creeks, or other rapid flow conveyances, the soil particles may be suspended in water currents for miles until reaching the pond where settling will take place. A well vegetated wetlands above the pond may intercept much of the soil load by causing the flow to spread and lose velocity.

Dam Erosion- Filling is not the only threat to the integrity of the system. Many ponds are a containment created by damming, and water currents exiting the pond can likewise erode the dam or spillway. Surface erosion on a spillway may occur on the surface where water velocity is greatest, and at the distal end where water falls away from the spillway. It also may be hidden in subterranean "worm holes" that penetrate the dam or burrow under protective surfaces of the spillway. These can be extremely difficult to stop once water has penetrated the designed barrier. Mitigation measures include the proper soil types in dam or spillway construction, and avoidance of culverts or trees which penetrate and compromise the dam integrity.