GEOLOGY

Detailed explanation-1: -Downcutting is a form of erosion. The lowest elevation that a river channel can erode is referred to as ultimate base level. For rivers that empty into an ocean, ultimate base level is equal to sea level. At this elevation, the gradient (slope) of the river is equal to zero.

Detailed explanation-2: -Streams erode their channels through a mix of abrasion, grinding, and dissolving materials. Solid materials forming the stream’s channel can get worn down by the flowing water. They can also be pressed against each other by the stream, or become part of the chemical solution of the stream.

Detailed explanation-3: -There is a theoretical limit to how low a river will erode. This theoretical limit is called the base level. A river will not erode below the base level. At base level, the gradient is zero, or horizontal.

Detailed explanation-4: -Base level is the lowest point to which a stream can erode it’s channel.

Detailed explanation-5: -A base level erosion is an imaginary surface of irregular shape, inclined toward the lower end of the principal, or trunk, the stream of a basin, under which the stream and its tributaries were presumed to be unable to erode.