Chapter 15 – River Systems
Water is a vital resource for all life. Currently there is ongoing research to see if there is some form of water on the Moon, but due to the limited amount of liquid freshwater on Earth, it must be recycled. The three major processes in the water cycle are condensation (changing from a gas to a liquid), precipitation, and evaporation (changing from a liquid to a gas). Evaporation requires a tremendous amount of energy to break the polar bonds of the water molecules, and condensation releases energy.
The amount of water a particular area receives and uses is described in what is called a water budget.
In this chapter we will study surface freshwater. We will study water in the atmosphere, the ground, and in the oceans in later chapters. Most freshwater on the surface is found in rivers, streams, and lakes. Virginia has only two natural lakes. They are Lake Drummond in the Great Dismal Swamp (southeastern VA), and Mountain Lake (near Blacksburg).
A river and all the tributaries which enter the river is called a river system. The land area drained by a river system is known as a watershed. Fishersville is in the Christians Creek watershed. Since Christians Creek flows into the Middle River, which flows into the South Fork of the Shenandoah, which flows into the Potomac; we are also part of the Potomac River watershed. The high point separating watersheds is a divide. The Continental Divide separates the continent into two major watersheds – the area draining into the Atlantic and the land draining into the Pacific.
The depression the river flows in is called its channel. The higher elevation where the river begins is known as its headwaters. When a river erodes the area where the stream begins, this is called headward erosion. Sometimes a stream erodes the divide between it and an adjacent river system, and this results in stream piracy, where the neighboring river now empties into the river system that eroded the divide.
The materials carried by a stream are called its load. There are three types of load: bed load is the material that bounces along the bottom, suspended load is the smaller material flowing along in the water, and dissolved load is the material that is in solution. When a stream carries a lot of load, sometimes a braided stream results as the stream’s load is deposited in a mound large enough to divide the stream into numerous channels.
The amount of water moving past a point along the river in a given period of time is known as discharge. Discharge is calculated by measuring the width (m) and average depth (m) of the river, then multiplying these to get cross-sectional area (m2). Next, measure the time (sec) it takes an object to float down the river a measured distance (m) to get velocity (m/sec). Finally, multiply the cross-sectional area (m2) by the velocity (m/sec) to get discharge (m3/sec).
Rivers can be grouped as “old” or “young.” Youthful rivers are fast-flowing, have a steep gradient, narrow V-shaped valleys, and may have rapids and waterfalls. Old rivers are slower moving, have broad floodplains (the area along a river covered with water during floods), have many bends our curves (called meanders), and sometimes form ox-bow lakes. After repeated flooding, a ridge of material called a natural levee is formed along the river banks. Manmade levees are built along some major rivers to control flooding, but this often results in more severe floods.
The faster a river is flowing the larger size and quantity load it can carry. When a river flows from a steep slope to a more gentle slope, its velocity decreases and its load is deposited in a fan-shaped feature called an alluvial fan. When a river empties into a larger body of water such as a bay or ocean, it again slows down and deposits its load. This feature is called a delta.