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Factors That Influence Water Movement in River Systems

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

River systems are complex and diverse. In this lesson, we will explore four main factors that influence the behavior, shape, and movement of water through a river.

Rivers

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?

Climate

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.

An ephemeral river is often dry
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Flow Velocity

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.

Flow velocity can be impacted by both gravity and friction
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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.

Stream Cross-Section

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.

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