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Things That Influence the Gradient of a River

A river is one of the most visibly dynamic components of the hydrological cycle that transfers water molecules through a relentless web of vapor, ice, freshwater and ocean. On a basic level, streams and rivers collect runoff from the Earth's surface and funnel it, ultimately, to the sea, in the interim eroding and depositing to build, destroy and modify landforms. Gradient is a measure of a river-course steepness, and it reflects various geological, hydrological and even biological influences.

Slope
- A young river cutting into an uplifted landscape or a head-water stream draining mountainous country experiences high gradient in its flow because of the severe slopes of its watershed. That gradient grants the watercourse down-cutting power, resulting in significant rates of erosion. By contrast, a big, mature river in level country begins to meander across a broad floodplain, depositing sediments along point bars and during inundations of its low-lying surroundings.
Uplift
- The Colorado Plateau showcases striking examples of entrenched meanders.
  
  Even a mature stream winding its way sluggishly across a flat floodplain can find itself rejuvenated, as geologists say, with a change of sea level. This could come in the form of an ice age, when the increase of continental and alpine glaciers and ice-sheets locks up more water in frozen form, lowering the level of the oceans. More commonly, tectonic activity -- a collision of tectonic plates, for example, or severe faulting -- raises some block of land, resulting in renewed down cutting on the part of the river. Uplift of the floodplain of a mature river might produce a deep, precipitous gorge, rendering the floodplain into an inundation-free terrace above the lip of the gorge. Entrenched meanders exist in spectacular form on the Colorado Plateau of the southwestern U.S., may stem from a relatively slow, measured uplift, where a mature river has the ability to erode while maintaining its meanders.
Bedrock
- Through its course, a river often flows over different layers of bedrock, the contrasting resistance of which can affect its gradient. A down-cutting river building a broad valley might encounter a buried belt of durable rock that erodes more slowly than surrounding layers. In such a case, a waterfall may result as points downstream of the resilient bedrock are eroded more quickly, and the hard layer is undercut.
Other Influences
- The dam-building activities of beavers strongly impact stretches of river.
  
  Biological phenomena may exert an influence on a river gradient. One example is the industrious activity of one of the world's largest rodents, the beaver, which inhabits wetlands of the Northern Hemisphere. Beavers will dam a fast-moving creek or river with felled logs, mud and other materials to create a backwater with the depth and dimensions amenable to their foraging and shelter. Where once a mountain stream tumbled through rapids, a low-gradient swamp or small lake now stands; over time, if the beavers move on, the body of water may gradually fill in to form a meadow or shrub land.

Nature