Chris has a master's degree in history and teaches at the University of Northern Colorado.
History lessons spend a lot of time talking about the importance of oceans and the people who cross them - but it's about time that rivers got their due. Rivers are extremely important features of a landscape, combining ground water, rain, and other forms of runoff into fresh, accessible water that feeds ecosystems. Rivers are awesome, but to fully appreciate them we need to understand the factors that contribute to river movement. What is it that makes them go with the flow?
There are four main factors we're going to cover, the first of which is climate. River systems behave differently based on the weather, and in particular on the amount of rainfall that contributes to the river. Major rivers, like the Mississippi or Nile, are defined by a perennial flow, which means that they have enough access to groundwater or rain to maintain a consistent level throughout the entire year. Rivers with less consistent sources of input may have intermittent flows, which means they are only full about 50% of the year. Finally, in drier climates where rain is scarce and groundwater is deeply buried, many rivers have ephemeral flows. These rivers only have water for a short period of the year, mainly during and after rain, and are often entirely dried up.
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Now, what we most often directly measure in a river is its flow velocity, or how fast the water within a river system moves. Flow velocity is determined by two things. First is gravity. A river that is fairly flat is likely to have a slower flow velocity than one which is on a steep hill. The second factor is friction. Water moves most quickly when it has less resistance, so the friction of water against rocks slows it down. For that reason, the fastest part of a river tends to be in the center, just below the surface. This is where friction is lowest. Along the bottom and sides, the water flow is resisted by the rocks or soil that makeup the river bed.
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Flow velocity is one of the most significant factors in the shape and behavior of a river system. Faster rivers tend to cut more deeply into the earth, and tend to be straighter thanks to the pressure from the flow. Slower rivers tend to meander, bending and curving as they meet resistance.
From here we need to look at the cross-sectional shape of a river. Imagine cutting a rectangular chunk out of the river and measuring it. The cross-sectional area (A) can be found by multiplying the width (W) and depth (D) of the channel. So, A= W * D. Next we need to know the wetted perimeter, or the amount of the riverbed that is generally in contact with water. We find that by multiplying the depth by two, and adding it to the width (WP= W + 2D).
Why do these equations matter? What this shows us is the ratio between the shape of the river, and the water moving through it. Rivers with a much larger cross-sectional area than wetted perimeter have water that flows faster and more freely, since less of it is in contact with the sides and bottom of the river. Remember, flow velocity is highest when the water has to overcome the least amount of resistance.
Our last major factor on river system behavior is sediment load, which is simply the amount of rocks and soil in a river. A river with soil or minerals that are completely dissolved has a dissolved load. This creates little friction, allowing the river to move unimpeded. A river with fine-grained sediment has a suspended load. This is why rivers get that muddy look. This load may slow the flow down a bit, but is not heavy enough to cause extra erosion.
Finally, we've got the bed load, or the heavy and course materials like pebbles and rocks. The bed load is too heavy to be permanently suspended in water, so it bumps along the bottom and sides. This can erode the riverbed, and also change the shape of the river if enough material accumulates. Have you ever noticed that when rivers bend, there tends to be a large pile of rocks along the curve? The water deposits rocks on these edges, but does not have enough strength to move them. But that's okay. Rivers can adapt to these obstacles and carve out more dramatic curves to compensate. They just go with the flow.
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River systems are influenced by a variety of factors. The climate can determine how much rain and groundwater contributes to a river, giving it either a perennial, intermittent, or ephemeral flow. The flow velocity, or speed of flow, can influence the shape and rate of erosion of a river system. The cross-sectional shape of a river dictates how much friction will impact the flow of water within a river. Finally, the sediment load, or the amount of rocks and soil in the river, impacts its flow velocity and shape. Together, these forces help define river systems and dictate how the river flow will go.
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