Req 4 — Watersheds, Basins & Aquifers

This requirement is about reading the landscape. You will define a watershed, trace drainage areas on a contour map, and connect surface water to larger river basins and underground aquifers.

Requirement 4a

A watershed is all the land area that drains water to the same place. That place might be a ditch, creek, pond, river, lake, or estuary. If two raindrops land on opposite sides of a ridge and flow to different streams, they are in different watersheds.

You can think of a watershed like a giant bowl. The ridges and high ground form the edges. Everything inside drains toward a common low point.

Requirement 4b

The smallest watershed on a map is usually the drainage area feeding a tiny stream, draw, or hollow. To outline it, find the low point where water exits the area, then trace the surrounding high ground that would divide runoff from neighboring areas.

A contour map helps because contour lines show elevation. Water flows downhill and crosses contour lines at roughly right angles. Watershed boundaries usually follow ridges, not valleys.

A simple tracing method

1. Find the stream or drainage line you want to study.
2. Mark the point where water leaves the smallest area you can identify.
3. Look uphill on both sides for the ridges.
4. Connect the high points so your line stays on the divide between one drainage and the next.

Requirement 4c

This part teaches an important idea: watersheds are nested. A tiny tributary watershed sits inside a larger creek watershed, which sits inside a still larger river watershed.

When you outline the next larger watershed, you are showing how small drainage systems fit into bigger ones. That is why land use on a seemingly small hillside can matter far away. The little stream drains to a bigger stream, and the bigger stream drains to a river.

Requirement 4d

A river basin is a large area of land drained by a river and all its tributaries. In the Soil and Water Conservation pamphlet, the idea is framed this way: all river basins are large watersheds, but not every watershed is large enough to be called a river basin.

That matters because people in one part of the basin are connected to people elsewhere in the basin. Soil washed off fields upstream can become sediment downstream. Fertilizer or waste released near one town can affect water treatment for another town. Forest clearing in the headwaters can change flood risk far away.

Requirement 4e

An aquifer is a layer of rock, sand, or gravel underground that stores and transmits water. It is not usually an underground lake or cave full of open water. More often, the water sits in tiny spaces between particles or in cracks in rock.

Aquifers matter because many communities use wells as part or all of their drinking-water supply. Aquifers can also feed springs and streams. If an aquifer is polluted, that contamination can be hard and expensive to remove. If groundwater is pumped faster than it recharges, wells can decline.

In the next requirement, you will follow water through the hydrologic cycle and see how vegetation, land use, and industry affect supply and runoff.