Watersheds & Water Resources
- Define a watershed and describe how divides direct the flow of water
- Distinguish surface water from groundwater and describe aquifer recharge
- Calculate the discharge of a stream from its cross-section and flow velocity
What a watershed is
A watershed (drainage basin) is all the land area that drains its water — from rain, snowmelt, and streams — into a single common body of water, such as a river, lake, or bay. The high ground separating one watershed from the next is a divide (ridge); rain falling on one side flows to one river system, and rain on the other side flows to a different one. Because everything upstream eventually flows downstream, a pollutant released anywhere in a watershed can travel to the shared water body, which makes watersheds the natural unit for managing water quality.
Surface water and groundwater
Freshwater exists in two connected forms. Surface water sits on top of the ground in rivers, lakes, streams, and wetlands. Groundwater fills the spaces in soil and rock underground, stored in porous, water-bearing layers called aquifers. The top of the saturated zone is the water table. Water enters an aquifer through recharge — precipitation infiltrating downward — and leaves through discharge to springs and streams or through wells. When water is pumped from a well faster than it recharges, the water table drops and the aquifer is depleted, as is happening to the vast Ogallala Aquifer beneath the U.S. Great Plains.
Impervious surfaces and runoff
What covers the land changes how water moves. Forests and wetlands let rain infiltrate slowly into the ground, recharging aquifers and filtering the water. Replacing them with impervious surfaces — roads, roofs, parking lots — blocks infiltration, so rain becomes fast surface runoff that picks up pollutants and rushes into streams. This raises flood risk, lowers groundwater recharge, and degrades water quality downstream. Managing a watershed therefore means managing land use across the whole basin, not just the river itself.
A stream channel is 5 m wide and 2 m deep, and the water flows at 3 m/s. What is the stream’s discharge in cubic meters per second?
- 1.Find the cross-sectional area: A = width × depth = 5 m × 2 m = 10 m².
- 2.Write the discharge equation: Q = A × v.
- 3.Substitute: Q = 10 m² × 3 m/s.
- 4.Multiply: Q = 30 m³/s.
A factory dumps a pollutant into a small creek in the upper part of a watershed. Which statement best predicts the consequence?
Think of a watershed as a bathtub: the rim is the divide, and every drop that lands inside the rim heads for the same drain (the outlet river or lake). Whatever you spill anywhere in the tub ends up at that drain.
Groundwater is being pumped from the Ogallala Aquifer far faster than precipitation can recharge it. What is the most direct consequence?
For discharge problems, remember Q = A × v and that area = width × depth. Keep units consistent (meters and seconds give m³/s). A frequent follow-up asks how discharge changes if the stream gets deeper or faster after a storm — both raise Q proportionally.
Answer the 2 checkpoints as you read.
Sign in to save your progress