Running water is the most powerful agent of erosion. Continents are eroded primarily by running water at an average rate of 1 inch every 750 years. The velocity of a stream increases as its gradient increases but velocity is also influenced by factors such as degree of turbulence, position within the river, the course of the stream, the shape of the channel and the stream load.
Stages in the cycle of river erosion are labeled as youth, maturity and old age. Each stage has certain characteristics that are not necessarily related to age in years - only phases in development. Typically, rivers tend to have old-age-type development at their initial mouths and youthful development at their upper reaches. So the three stages may grade imperceptibly from one to another and also from one end of the stream to the other.
The youthful stage is characterized by rapid downcutting, high stream gradient, steep-sided valleys with narrow bottoms and waterfalls. The mature stage is characterized by a longer, smoother profile and no waterfalls or rapids.
Gradient is normally expressed as the number of feet a stream descends each mile of flow. In general, a stream's gradient decreases from its headwaters toward its mouth, resulting in a longitudinal profile concave towards the sky.
The base level of a stream is defined as the lowest level to which a stream can erode its channel. An obstacle such as a resistant rock across a stream can create a temporary base level. For example, if a stream passes into a lake, it cannot erode below the level of the lake until the lake is destroyed. Therefore different stretches of a river may be influenced by several temporary base levels. Of course the erosive power of a stream is always influenced by the ocean which is the ultimate base level below which no stream can erode. Many streams in Idaho eventually reach the ocean through the Columbia River.
If the base level is raised in some manner such as by a landslide blocking a stream, the stream's velocity is reduced and it can no longer carry as much material. Sedimentary material will then be deposited in the lake formed by the landslide. Conversely, if the base level is lowered, the stream will begin eroding its channel downward.
Running water transports material in 3 ways: solution, suspension and by rolling and bouncing on the stream bottom. Dissolved material is carried in suspension. About 270 million tons of dissolved material is delivered yearly to the oceans from streams in the United States. Particles of clay, silt and sand are generally carried along in the turbulent current of a stream. Some particles are too large and heavy to be picked up by water currents, but may be pushed and shoved along the stream bed.
Waterfalls are a fascinating and relatively rare occurrence. Waterfalls may be caused in several ways. For example, where a relatively resistant bed of rock overlies less resistant rock, undermining of the less resistant rocks can cause a falls. Waterfalls are short-lived features in the history of a stream as they are created by a temporary base level. As time passes, falls may slowly retreat upstream, perhaps as rapidly as several feet per year. There are many spectacular waterfalls in Idaho, including the 212-foot-high Shoshone Falls in the Snake River Canyon just north of Twin Falls.