Surface Water Explained

Geologists divide landscapes into their different drainage basins. Other names for a basin are watershed or catchment. All precipitation in a catchment drains into the same stream. There are drainage divides that separate one basin or catchment from another. As streams travel further, they come together to make rivers that drain as tributaries into other rivers. Finally, all rivers or continental waters from them will end up in a larger area, such as an ocean. Figure 58 depicts a basin and its main features:

Streams start as headwaters. These are often small tributaries or branches of larger streams or rivers to make a trunk stream. The mouth of the stream is where it ends and discharges its contents into something else. Two equal streams meet at a confluence. Many streams and rivers have a mouth in the ocean, which is where fresh water meets ocean water. Some do not ever reach the ocean. What then? These are closed basins or endorheic basins that end in a lake that just evaporates over time. The Great Salt Lake in Utah is an endorheic basin lake.

You can describe basins in several ways. A basin is also what we call the entirety of land that drains into the endpoint, which is usually the ocean. Most of the Midwest and some of the eastern parts of the US are of the same basin. The west and part of the east are in their own basins. The Great Salt Lake area and several others in Eurasia and parts of Africa have their own endorheic basins.

SURFACE WATER EXPLAINED

Streams are flowing bodies of water. In geology, the terms brook, creek, and river are not used. Everything is a stream. Streams cause a great deal of erosion all the time and send sediment downstream toward the oceans. There are two factors that affect the degree of erosion. The first is the velocity of the stream's water. How fast is it going? Second, you want to know the channel gradient, which is how downsloping the water is. It is measured in terms like feet per mile or meters per kilometer. A taller mountain with a stream probably has a larger channel gradient.

Erosion of a stream leads to a valley over time. The wider the valley, the larger was a given stream at one point – even if it is narrow in the present time. The velocity or water speed you see in a stream depends on several factors. The channel gradient, the

stream width, and the underwater terrain all determine the water speed. What's happening downstream also matters. If there are tributaries that are discharging water downstream, this will decrease the water speed to some degree. In most cases, the channel velocity increases as a stream gets wider, deeper, and has more tributary-fed water in it.

The discharge of a stream is the stream size rather than its width or length. This is the volume of water flowing past any given point in the stream. It is measured in cubic feet, or cubic meters. In other words, it is a volume unit per time. A smaller stream will naturally have less discharge in it. Obviously, the stream's discharge will increase as you go downstream.

So, if the stream gets shallower but stays the same width, the flow or velocity rate will increase as the same volume of water has a narrower area to squeeze through. Try this by putting your thumb on the end of a garden hose to see how this works. If you add more volume through a heavy rain or snowmelt, you will also get a bigger discharge and more velocity.

Streams tend to be curvy so that they are a lot like half tubes. The water velocity will not be the same throughout. At a bend in the curve, you will see higher velocities at the outer edge of the curve. As the stream narrows, the highest velocity will be at the top and middle section – far away from friction near the bottom or edges.

Geologists refer to a line called the thalweg of a stream, which is a stream's natural progression and its deepest channel. If you draw a line down the stream at its deepest depths, you will get the thalweg. Wherever there is water away from the bend and away from friction, you'll get faster speeds. The thalweg does not have to be underwater at a given point if it's part of the same valley.

The pattern of all the tributaries in any region or area is the drainage pattern. There are many things that determine what this looks like, including the type of bedrock and whether there are local faults or folds to consider. As you study a geological area, keep these different patterns in mind as they will help you understand what the earth looks like beneath the watery areas: