Seasonally, winter brings cold temperatures, snow, and ice. Surface waters tend to have less dissolved oxygen, lower concentrations of suspended sediment, and higher concentrations of nutrients than during other seasons. Ammonia and dissolved phosphorus concentrations can be high under ice conditions. Snowmelt and precipitation runoff delivers nutrients, pesticides , and sediment to streams. Soil preparation and the application of chemicals relative to the occurrence of precipitation accounts for some of the variability in the amount of contaminants that reached Study Unit streams.
Summer brings warm temperatures, thunderstorms, and generally declining water levels in rivers.
The periodic rainstorms increase suspended sediment and transport pesticides applied in spring and summer to surface waters. Fall brings cool temperatures, falling leaves, and low stream-water levels. Reduced flows generally correspond to reduced suspended-sediment, nutrient, and pesticide concentrations. More pesticides were detected for the Otter Tail study and at higher concentrations than for the Sheyenne Delta study. Although land use in both areas was similar, the Otter Tail aquifer received more recharge because of coarser textured soils and generally more irrigation.