Why do rivers become wider and deeper downstream?
There are two fundamentally different types of rivers, related to how they interact with the groundwater system. Effluent rivers are streams which get their water from the groundwater. The surface of the stream directly relates to the surface of the groundwater (called the water table), and the stream will rise and fall as the water table rises and falls. Effluent streams are common in temperate to tropical climates, and generally become both wider and deeper downstream due to increased discharge: the continual addition of water from tributary streams (and their groundwater sources). Effluent streams also commonly run year round, again because of their relationship to the groundwater. Good examples of effluent streams would be the Mississippi River, the Amazon, and the Columbia here in the Pacific Northwest. In nearly every case effluent streams end up at the beach, and, in keeping with the Third Law of GeoFantasy, are responsible for delivering the flotsam and jetsam of the earth to the ocean.
The second type of stream is called an influent stream.These are far less common, and are most commonly found in arid climates. Influent streams do not usually get deeper and wider downstream. In fact, most actually lose water as they flow towards their ultimate destiny. There can be several reasons for the loss of water. Seepage into the ground (adding water to the groundwater system), evaporation, and use by plants and animals are common causes of discharge reduction. Some actually lose so much water that they dry up completely and never make it to the ocean at all. Good examples of influent streams include the Nile, and the Colorado River in the southwestern United States.
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