The area over which snow and rain fall that provides a stream with its water is called its drainage basin (or watershed). The watershed includes the stream and all its tributaries as well as the adjacent hill slopes that drain into these tributary streams. Its root, or base, is the point where the stream exits the watershed.

Click the "Show Watershed" button. The red line (a divide) separates one watershed from another.

Click the button again to see a grid. You are going to use this grid to measure the area of the watershed. Here, the vertical and horizontal grid lines have a spacing of 0.25 miles. Do you know how to calculate the area of each cell of the grid?

To estimate the area of this watershed you could simply count the number of grid squares it contains and multiply by the area of each square. Here, the computer will help you.

Click once in a square if it's mostly (90% or more) within the watershed. Click twice in a square if it is partially (about 10% - 90%) inside. You can keep clicking in squares to change how they are counted. A square that is dark gray counts as a whole square and a square that is light gray counts as a partial square ( squares partially "in" contribute 1/2 as much to the total area as squares completely "in"). You can click and drag to fill in multiple squares. The total area is based on how many squares are filled in as either light or dark gray.