13.1 Streams and Rivers Key Idea: Streams and rivers carry a portion of the precipitation that falls on land back to the oceans.

Lesson Objectives Define a river system Describe the characteristics of a stream or river that affects its ability to erode sediments.

River Systems A permanent body of running water is called a stream or a river. Rivers carry more water than streams. A small stream may run into a larger stream; the larger stream may run into a river, and so on. A small body of water which runs into a larger one is called a tributary.

A river system

La Plata River System_ South America

River Systems Usually a large river collects all the smaller rivers in a geographical area. This large river and all its tributary form a river system. The surface covered by a river system is called a drainage basin or watershed. The high land that separates one river system from another is called divide.

Drainage Basin or Watershed

Mississippi Watershed is the largest in the US

Characteristics of Streams and Rivers The ability of a stream or river to erode and transport sediments is affected by many factors. The most important factors are: Velocity Gradient Discharge Channel

Velocity The velocity of a stream or river is the amount of space the water travels in a given amount of time. A fast running river has more kinetic energy, therefore it erodes materials more quickly and can transport larger particles than a slow moving river. The velocity of a river is determined by: the steepness of the slope The amount of water traveling downstream The shape of the path through which the water travels.

Velocity, Illustrated

Gradient The steepness of the slope of a stream or river is called gradient. A river’s gradient varies along its course. A river flowing up the hill on steep slope, has a high gradient. A river flowing on a flat plane, with a very small incline, has a very low gradient.

Discharge The discharge of a river is the amount, or volume of water that passes a certain point in a given amount of time. In many rivers the discharge increases downstream , because the tributaries continually add more water. In the desert, discharge may decrease downstream. Discharge varies throughout the year: during increased precipitation, or when the snow is melting, the discharge increases. The velocity of the river also increases. Rivers become wider, and deeper, and floods are possible.

Discharge Increases Downstream

River Channel The path through which the water flows in a stream or river is called its channel. The size and shape of the channel affect the velocity of the water. The channel of a shallow, winding stream with many boulders has a great deal of surface area in contact with water flowing through it; friction slows the water down. A straight channel that is wide and deep has les surface area in contact with the water, so the water’s velocity is greater.

Boulders increase friction and decrease the velocity of the river