# Water dam would fill up with sediment deposits.

Water stops flowing when it reaches its lowest level; this is called the base level. Learn about the base level of a stream, how it affects erosion, and some examples of local base levels, including lakes, dams and waterfalls.

## Base Level

If you go to the park and watch children playing on the slide, you’ll notice that no matter how steep the slide is, the kids slow down as they approach the bottom.

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This is because the bottom of the slide flattens out. If you were to extend the flat bottom of the slide a few more feet, the children’s velocity would slow so much that their forward progress would come to a halt. You could say that they reached the lowest level of the slide, and this means less movement or activity.If we were talking about a stream, this lowest level would be called the base level. The base level of a stream can be defined as the lowest level to which running water can flow. At the base level, the water in the stream has less velocity, which means the water flow has less energy, so its ability to erode or chip away at the land surrounding it is decreased. This inability to erode is important; in fact, let’s tack it on to the end of our definition.

Our updated definition would read ‘the base level of a stream is the lowest level to which running water can flow and erode.’

## Ultimate Base Level ; Local Base Level

Now, if you really think about this, the lowest level that water can flow would have to be sea level, right? After all, sea level is the height of the ocean surface, and the ocean is the biggest body of water there is. Well, this is partly right.

Sea level is referred to as the ultimate base level, because all streams, rivers and waterways eventually erode toward that ultimate destination.But there is another type of base level that can be considered a temporary base level, because water gets stuck or stalled there for a period of time. This is called the local base level, and we can define it as the level where the velocity and eroding power of the water is temporarily lost.

Of course, we’re talking temporarily in geological time, so this could mean decades or centuries or even longer.

## Examples of Local Base Levels

Let’s look at examples of local base levels. If a stream flows into a lake, the lake becomes the local base level for that stream. The velocity of the running water of the stream slows as it hits the bigger body of water, kind of like your ability to walk fast diminishes as you hit a crowd of people. This lowered velocity leads to less erosion of the stream’s channel.Another example of a local base level is a dam. If you were to take a truckload of boulders and drop them into a stream, they would stop the flow of water.

The water behind your dam would pool and slow the velocity of the water flowing toward the dam. The dam created a new local base level for the stream. This dam could last for a long time, but if you left this area alone for long enough, the calm waters, or reservoir, behind the dam would fill up with sediment deposits. Eventually, this would build up to the point where the water would be able to flow over the dam, eroding the land downstream as the stream makes its way to the ultimate base level, the sea.A waterfall allows some water to flow over top of it, but it is another example of a local base level.

The top of a waterfall is made of tough, water-resistant rock. The running water has a hard time eroding this rock, so the water behind the rock ledge gets stuck and slows its flow. The result is a temporary base level, and we see that the lowest level of erosion behind the waterfall becomes the level of the resistant rock. Of course, with a waterfall, some water makes it over the top and crashes down below. This water has a lot of velocity and can continue to erode as it travels downstream.

## Lesson Summary

Let’s review.

Base level can be defined as the lowest level to which running water can flow and erode. There are two types of base levels. The ultimate base level can be thought of as sea level because all rivers and waterways eventually erode toward that ultimate destination. The local base level can be defined as the level where the velocity and eroding power of the water is temporarily lost.

Examples of local base levels include a lake, a dam, and a waterfall. When streams flow into these areas, the stream has reached the lowest level at which it can flow. This robs the water of its energy and its ability to erode.

## Learning Outcomes

You should have the ability to do the following after watching this video lesson:

• Define base level in terms of a stream of water
• Identify what the ultimate base level is
• Explain what local base levels are and provide examples of them
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