The running water in a stream will erode (wear away) and move material within its channel, including dissolved substances (materials taken into solution during chemical weathering). The solid sediments may range in size from tiny clay and silt particles too small for the naked eye to view up to sand and gravel-sized sediments. Even boulders have been carried by large flows. The smaller particles kept in suspension by the water’s flow are called suspended load. Larger particles typically travel as bed load, stumbling along the stream bed (Figure 5.1). While the dissolved, suspended, and bed loads may travel long distances (ex. from the headwaters of the Mississippi River in Minnesota to the Gulf of Mexico at New Orleans), they will eventually settle out, or deposit. These stream deposited sediments called alluvium. Although alluvium can be deposited at any time, it most often occurs during flood events. To more effectively transport sediment, a stream needs energy. This energy is mostly a function of the amount of water and its velocity, as more (and larger) sediment can be carried by a fast-moving stream. As a stream loses its energy and slows down, material will be deposited.
Under normal conditions, water will remain in a stream channel. When the amount of water in a stream exceeds its banks, the water that spills over the channel will decrease in velocity rapidly due to the greater friction on the water. As it drops velocity, it will also drop the larger sandy material it is carrying right along the channel margins, resulting in ridges of sandy alluvium called natural levees (Figure 5.2). As numerous flooding events occur, these ridges build up under repeated deposition. These levees are part of a larger landform known as a floodplain. A floodplain is a relatively flat land adjacent to the stream that is subject to flooding during times of high discharge (Figure 5.2).
Stream Drainage Basins and Patterns
The drainage basin of a stream includes all the land that is drained by one stream, including all of its tributaries (the smaller streams that feed into the main stream). You are in a drainage basin right now. Do you know which one? You can find out on the internet. Go to the Environmental Protection Agency’s webpage (epa.gov) and search for Surf Your Watershed to find out. The higher areas that separate drainage basins are called drainage divides. For North America, the continental divide in the Rocky Mountains separates water that drains to the west to the Pacific Ocean from water that drains to the east to the Gulf of Mexico.
As water flows over rock, it is influenced by it. Water wants to flow in the area of least resistance, so it is attracted to softer rock, rather than hard, resistant rock. This can result in characteristic patterns of drainage. Some of the more common drainage patterns include:
- Dendritic – this drainage pattern indicates uniformly resistant bedrock that often includes horizontal rocks. Since all the rock is uniform, the water is not attracted to any one area and spreads out in a branching pattern, similar to the branches of a tree.
- Trellis – this drainage pattern indicates alternating resistant and nonresistant bedrock that has been deformed (folded) into parallel ridges and valleys. The water is attracted to the softer rock and appears much like a rose climbing on a trellis in a garden.
- Radial – this drainage pattern forms as streams flow away from a central high point, such as a volcano, resembling the spokes in a wheel.
- Rectangular – this drainage pattern forms in areas in which rock has been fractured or faulted which created weakened rock. Streams are then attracted to the less resistant rock and create a network of channels that make right-angle bends as they intersect these breaking points. This pattern will often look like rectangles or squares.
- Deranged – this drainage pattern does not follow the rules. It consists of a random pattern of stream channels characterized by irregularity. It indicates that the drainage developed recently and has not had time to form one of the other drainage patterns yet.