A reach in geography can mean several things. Most generally, a reach is any length of a stream between any two points. The points may be selected for any reason, such as gauging stations, river mile

River mile

In the United States, a River mile is a measure of distance in miles along a river from its mouth. River mile numbers begin at zero and increase further upstream. The corresponding metric unit using kilometers is the River kilometer...

s, natural features, and topography. They may be arbitrary or vague.

A reach may also be an expanse, or widening, of a stream or river channel. This commonly occurs after the river or stream is dammed. A reach is similar to an arm

Arm (geography)

In geography, an arm is a narrow extension, inlet, or smaller reach, of water from a much larger body of water, like an ocean, sea, or lake. Although different geographically, a sound or bay may be called an arm....

.

The term "reach" can also refer to:

• An extended portion or stretch of land or water;
• a straight portion of a stream or river, as from one turn to another;
• a level stretch, as between locks in a canal
  Canal
  Canals are man-made channels for water. There are two types of canal:#Waterways: navigable transportation canals used for carrying ships and boats shipping goods and conveying people, further subdivided into two kinds:...
  
  ;
• an arm of the sea extending up into the land.