But rivers that course down domes generally flow in a ring-like arrangement known as an annular pattern . The picture above is modified from a map of South America. The picture at left depicts a trellis pattern . The gray streak represents a hard rock ridge that rivers flow along. When a river becomes big and strong enough, it may erode through the ridge, forming a gap.
Streams flow down the slopes of a volcano at right, forming a radial pattern .
If you flew over the volcano in an airplane, the streams might resemble a bicycle tire or an octopus. Rivers in Action Many rivers begin as melting snow high in scenic mountains, often leaping to life as rocky rapids or cascading waterfalls. As they enter lowlands, rivers begin to run more slowly, growing in size as they re fed by other rivers and streams. They generally become a little less clear as their load of silt grows. Rivers never flow in a perfectly straight line.