The demand for water resources for recreation, agriculture, and industry is increasing. Many kinds of plant and animal aquatic pests can interfere with water uses. Control of aquatic pests must be done without harm to people and the environment.
Habitats for aquatic weeds involve various proportions of water and soil, including intermittently wet ditches, ditches which always hold standing water, streams, stock ponds, farm ponds, lakes, ornamental ponds, and intermediate habitats. This manual considers three types of water situations - static, limited flow impoundments, and moving water.
Static Water
Static water is confined for considerable periods of the year, or totally confined within a known area, with no downstream movement. However, even totally enclosed bodies of water often have appreciable water movement because of wind and changes in water temperature. Weeds commonly grow in static water up to 12 feet deep. Weeds may grow in very clear water that is more than 20 feet deep. If a herbicide is applied for weed control, there is no reason to expect that any appreciable downstream effect may occur, unless there is overflow resulting from unusual storm conditions.
Limited -flow Water Impoundments
Ditches may be intermittently wet or dry, depending upon climatic conditions. However, herbicides applied to these habitats may move downstream following an influx of water from surrounding areas. The purpose of the ditch is to drain the surrounding land area so considerable amounts of water must pass through it.
Many farm ponds may be characterized as having limited flow because there nearly always is an overflow pipe and an emergency overflow channel (spillway). The overflow pipe is designed to permit passage of a continuous and relatively well-defined amount of water at all times. The emergency spillway is provided to release from the pond when storms dump in excess amounts of water in a short time. In these situations, small amounts of pesticides may be carried downstream from the application site. Larger amounts may be found downstream after sudden rain storms, which interrupt or come immediately after pesticide application.
Moving Water
Moving water is found in small streams, creeks, streams, and rivers where there is always some detectable downstream current. Applied pesticides may be found in downstream locations in varying amounts away from the area of original application. Such situations present the greatest potential for concern as an environmental hazard.