When cleaning up water pollution in a large lake or river, it is important to clean up the entire? ?

When cleaning up water pollution in a large lake or river, it is important to clean up the entire watershed. It is a basin-like landform defined by highpoints and ridgelines that descend into lower elevations and stream valleys. This is because pollution can be transfered to other areas of the lake or the river when this part is not cleaned well.

Dear where is the map?