Updated: Oct 28, 2021
All land on earth is a watershed. All our activities play an important role in watersheds, yet few people understand them. Still fewer know how a watershed works or can describe the boundaries of the ones in which they live.
A watershed is the land area down which all water flows by networks of channels, that increase in size as the amount of water, sediment, and dissolved materials they must carry increases. Each watershed is an interconnected land-water system that conveys water to its outlet, either a larger stream, an inland lake, a wetland, an estuary, or the ocean. A watershed may be the drainage area surrounding a lake that has no surface outlet, or a river basin as large as that of the Columbia River. Even a puddle has its own watershed.
Water is a mover...
Water channels change by erosion and deposition. Natural channels of rivers increase in size downstream as tributaries enter and add to the flow. A channel is neither straight nor uniform, yet its average size changes in a regular and progressive fashion. In some cases, large floods cause new channels to form, leaving once-productive streams dry and barren.
"Into each life some rain must fall.”
The greatest factor controlling streamflow is the amount of precipitation that falls in the watershed as rain or snow. Sometimes some of the precipitation that falls in a watershed doesn't flow out, and sometimes a stream will continue to flow where there is no recent precipitation. When rain falls on dry ground, some of the water soaks in and infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging groundwater aquifers. Water may travel long distances or remain in storage for long periods before returning to the surface.
The amount of water that will soak in over time depends on the physical characteristics of the watershed.
Soil characteristics: Clay and rocky soils absorb less water at a slower rate than sandy soils. Soils absorbing less water results in more runoff overland into streams.
Soil saturation: Like a wet sponge, soil already saturated from previous rainfall can't absorb much more, so more rainfall will become surface runoff.
Land cover: Impervious surfaces, such as parking lots, roads, and developments, are a fast track for a watershed. Hard surfaces move water quickly into storm drains that drain directly into streams and flooding is more likely.
Slope of the land: Water moving on a steep slope runs off more quickly than water moving on flat land
The term “runoff” refers to the various processes that ultimately produce streamflow. Surface runoff occurs when the soil’s infiltration capacity has been exceeded and excess water on the soil surface travels down slope.
Land uses can have a major influence on the amount of precipitation that becomes streamflow. Decreasing the amount of vegetation on the land increases the amount of surface runoff. In contrast, abundant vegetation slows runoff, encourages infiltration, and consequently increases the amount of water stored in the watershed.
Surface runoff is the primary mechanisms for transporting nonpoint source pollutants from land to surface waters. The speed and depth of the runoff affects the amount of pollutants transported. Climate, geology, topography, soil characteristics, and vegetative cover all affect how quickly and how much runoff becomes streamflow.
Everything from the top of the mountains down to the flats at the bottom, are part of one system. They all depend on each other for their very existence. All living elements in the watershed interact with and modify the energy flow throughout the system. The different conditions, soil types, topography, vegetative cover, etc., define the specific character of each watershed. Rivers do not stop at state lines or national boundaries. The effects of natural and human processes in a watershed are focused at its outlet, wherever that may be. Each water shed is a part of a larger watershed whose downstream portion is affected by everything above it.