A
drainage basin, or watershed, is the total area that is drained by a river or stream. The stream itself is made up of a
trunk (main channel) and its tributaries.
Tributaries are smaller streams that flow into a larger one. A
meandering river is a single channel in a valley which has many curves. These curves develop because the land is flat, so the river cannot flow very fast – but at the same time there is more water to move. The way that the energy of this moving water is expended is by having the river curve around itself over and over – this makes the river longer, and thus able to accommodate the water. It is called a meandering river because over time, the river “wanders” back and forth across its floodplain (meandering means wandering). The outside of a meander loop is called the
cutbank, because the water cuts into it and erodes it back. The inside of a meander loop is called the
point bar, and this is an area where slower moving water drops sediment, building up a sandy bank over time. This slowly moves the meander loops into wider and wider forms. Sometimes these meander loops will erode far enough that they intersect. When this happens, the water in the river will start to flow preferentially into the straighter part of the channel, and the water in the loop slows down and begins to drop its sediment. Over time, the meander is cut off completely. A cut-off meander loop that still holds water is called an
oxbow lake. If the water eventually dries up and the oxbow fills with sediment, it is called a
meander scar. In this way, the river moves back and forth across the floodplain. The main feature of a
braided river is that it is made up of several channels interconnecting over and over within a valley. Alternately, you might think of it as a single channel with lots of
sand bars or
islands.
A few other concepts that are useful to know in river systems are gradient, velocity, discharge, base level and load.
Gradient is the slope of the river – or the vertical drop in elevation over distance. We generally express it as feet per mile or meters per kilometer. Gradient is usually steepest near the
headwaters (start of the river) and lowest near the
mouth (end of the river).
Velocity is how quickly the river is moving. We measure this as feet per second, or meters per second.
Discharge is the amount of water in a channel. We measure this using the formula Q=WxDxV/2. This is the width of the river, times the depth of the river, times the velocity; then the total is divided by two. Finally, the
load of a river is the sediment that the river is transporting. This is dependent upon the available sediment and how easy it is to erode and transport, as well as gradient, discharge and velocity. The maximum load a river can carry is called its
capacity.
LAB EXERCISES FOR WEEK 8
Chapter 12: Pages 179-194
·
Complete exercises 12.1, 12.2 (part a&b only), 12.3, 12.4, 12.6
NOTE: Be sure to read the lab background information in the module, as well as all information in the lab book before attempting the lab!
