Conversely, when the groundwater level lies below the stream bed, surface water will flow from the stream into the aquifer. In the Upper Chehalis watershed, ground-water discharge maintains streamflow during periods of little or no rainfall. Each of these activities can have profound effects on the amount and quality of water moving through a watershed. Logging operations can strip soils of vegetation, increasing the amount of runoff to streams. In agricultural areas, chemicals and livestock wastes can drain into streams.
Irrigated agriculture requires significant volumes of water.
Municipalities and industries use large amounts of water and may discharge waste water to streams. Stormwater runoff can also carry pollutants from these surfaces into streams. Increased runoff from impervious surfaces also contributes to flooding problems during winter storm events. Water quality is closely tied to water quantity. Water supplies must be of high quality for human uses and for fish and wildlife habitat.