The lush vegetation along watercourses is produced by the extra moisture seeping out from streams to supply sub-irrigation for plant growth along their banks. The streamside thickets or riparian zones, have an appearance and a microclimate very different from the surrounding rangelands. In addition to much heavier vegetation, there is more shade and higher humidity and increased air movement.
To live in shade, leaves of trees and other plants are broad and flat and are spread on wide branches to maximize collection of the suns rays.
The water for this unique environment comes from rainfall draining from the surrounding hills and from the mountains to the east. These hills and mountains are called a watershed. Water follows the ravines and low spots gradually working it way to the sea. There are several streams on the Bison Range that include Mission Creek, along the north side, and the Jocko River along the south side. Some of these streams drain a large area and flow all year. Others are seasonal, draining only small open areas with little vegetation to hold the moisture.
In flat areas, streams wander and bend, creating little marshes. Stream banks erode at the outside of the turns and silt on the inside, constantly changing the course of the stream. The speed of flowing water varies with the steepness of the terrain and the main current roams from bank to bank. Currents and eddies can create deep holes in otherwise shallow streams so there can be holes and drop-offs in any stream of flowing water.
Fish and insects and other creatures that live in flowing water are rapid swimmers or they are specially adapted to cling to, or hide under rocks. Insect life in streams is more abundant, but made up of fewer different species than live in still water. Little aquatic vegetation grows in swift streams.
A wide variety of birds and other wildlife, especially deer and small mammals such as mink, live along streams because of the excellent protective cover and wide variation of food sources to be found there.