Week 9
Rivers
Week 9
Rivers
Click
on the link below to hear a mini-lecture audio segment.
Introduction
It has been estimated that there are nearly 1.5 billion cubic kilometers of water associated with the Earth. Of this volume over 97% is contained in the ocean, and 2.7% is freshwater. The freshwater is partitioned between glacial and land ice (2.065%), groundwater (0.575), lakes (0.017%), atmospheric water (0.0001%), and rivers (0.001%).
In spite of the relatively small percentage of the total water on the earth they represent rivers have historically been the centers of civilization and development. Most of the early civilizations were centered around rivers such as the Nile and the Tigris and Euphrates. In part this is due to the fact that rivers are often the centers of drainage for entire continents. The number and size of the rivers varies considerably among the major continents.
Although rivers do not represent a large proportion of the Earth’s water,
the major rivers of the Earth are important factors in the sculpting of the
landscape. One of the important aspects of a river is the area it drains. The
drainage area of a river is the surface area of the Earth that is drained of
its surface water (and some groundwater).
Anatomy of a River
Rivers tend to start in mountainous or high slope areas. This part of the river is called the headwaters of the river. The river channel tends to be steep and V-shaped, and the bed is rough and irregular.
As the river exits the foothills, it flows into the plains. The gradient of the river in the plains tends to decrease rapidly, and the river channel broadens and deepens. The river bed is smoother and the path of the river often meanders.
Eventually the river flows into the base level, forming a delta. The delta is a feature that forms when the sediment carried by the river is deposited at the base level (sea level or a lake).
The Cross-section of a River
Longitudinal profiles, as shown in below, provide a useful way of looking at rivers. The slope, or gradient, of the river is highest near the headwaters and lowest at the mouth of the river, where it is terminated in a larger body of water, such as a river or ocean.
Flow in Rivers
Velocity. Stream velocity is simply the distance that the water in a river or stream travels over an interval of time, measured in ft/s or m/s. The velocity is related to the discharge of a river, the slope angle of the river bed, and the type of flow.
Laminar flow occurs when the water particles move along straight lines, called stream lines that are parallel to each other.
Turbulent flow tends to occur when the velocity of water and the vicosity and friction are low.
Let's look at a typical river through virtual reality.
Put on your 3D glasses
and travel back through time to the first trip down the Colorado River.
Gradient
As discussed previously, streams and rivers tend to flow from a source in a higher elevation, called the headwaters, downward towards a terminus, called a base level, at a lower elevation. The gradient of a stream is a measure of how far a river drops in elevation as it travels a certain horizontal distance. The gradient is the drop in elevation of the river, divided by the run, or horizontal distance the river has traveled, as shown in the equation below:
Gradient (m/km) = rise / run = elevation change / horizontal
distance
The gradient of a river tends to be highest at the headwaters; decreasing to essentially zero at the base level, as shown in the longitudinal profile.
Discharge
The discharge of a river is illustrated below. Typically the depth and width values of the stream that we use in the calculation of discharge are estimates that are the result of several measurements of depth (at several sites in the river) and width (at various depths) along a single imaginary line across the stream, perpendicular to the stream bank. In essence, we are determining an average width and depth that can be used to determine the cross-sectional area of the stream.
Stream Load
Streams typically carry large amounts of rock fragments and sediment as they move towards their base levels. This material is called the stream load, and it is composed of bed load, suspended load, and dissolved load.
Erosion and Deposition in Rivers
Rivers are reponsible for the creation and sculpting of most of the landscapes we observe on the earth. Rivers typically display three different behaviors.
They are:
Erosion, which is the breakdown and removal of material.
Transport, which is the movement of material down the river.
Deposition, which is the acucmulation of material along the banks and beds of a river.
Put on your 3D glasses
and view a 3D virtual field trip to a stream.
Other Fluvial Landforms.
Rivers and streams
often leave behind distinctive landforms or morphologies that are the result
of specialized conditions at certain places along the river. When a graded stream
approaches its terminus it no longer erodes downward and begins to erode laterally
creating a broad sinuous path called a meandering stream,
as shown in below. As meander grows more sinuous and the curve begins to double
back on itself, the stream erodes through the meander and becomes straight.
This process leaves behind an abandoned or cutoff meander, called an oxbow
lake, as shown in below. When streams deposit sediment within the stream
channel, sand and silt bars are created which cause the stream to split into
segments that form an intertwined network called a braided
stream, as shown in below.
Deltas
At the terminus of a river the water flows into a relatively low energy environment in which the sediment that has been transported by the river must be deposited. The depositional structure created is called a delta.
Deltas evolve with time and often migrate from place to place. In the last 5000 years the Mississippi River delta has migrated over 100 miles, as shown below. Most of the land in the states of Mississippi and Louisiana were created as the delta formed and moved outward into the Gulf of Mexico. If a delta is actively moving outward at its terminus, it is considered stream-dominated because the deposition of sediment by the stream is the dominant process. If the movement of the tides alters the delta and produces submerged delta deposits and islands, it is considered tide-dominated. In a tide-dominated delta the activity of the tides has more of an effect on the delta than stream deposition. The Mississippi River is a stream dominated delta, and the Chesapeake Bay area is an example of a tide-dominated delta.
Bars
Landforms may also be created by deposition of sediment within the stream. Bars are ridge-like deposits of poorly sorted sediment found near the banks of a stream or in the middle of the stream channel. The presence of bars indicates that at some time in the past the discharge of the stream was higher and coarser sediment was transported by the stream water. When the discharge decreased, the coarse sediment was deposited, forming a ridge-like structure usually parallel to the direction of flow. During a flood, all grain sizes of sediment might be transported down a stream that, under normal circumstances, might only transport silt or clay. When the flood recedes and the discharge is diminished, the coarse cobbles and boulders are deposited on the bed. This further slows the flow of water, resulting in the deposition of finer sand and silt among the boulders and cobbles. In the figure below the evolution of a bar is shown. At the beginning of the cycle (A) a stream is shown with a bar deposit along its bank, resulting from an earlier flood. Later (B), another flood occurs eroding and transporting the sediment in the bar downstream. After the later flood recedes (C), a new unsorted bar deposit is formed.
Drainage Basins
Every river system is provided with water from a variety of sources. These include tributary streams, surface runoff, and groundwater. The surface area that contains the tributaries and supplies the surface runoff that feeds the river is called the drainage basin. Typically, ridges that form a valley or bowl-shaped depression that channels the water towards the river bound the drainage basin.
Floods and Flooding
Floods are among the most difficult natural hazards to mitigate although they have occurred since the beginning of recorded history. Property damage in the U.S. from floods topped $12 billion in 1993, and the number of deaths due to flooding is the only natural hazard-related death figure to rise on an annual basis since 1940. Floods can destroy entire towns and cities. In 1931 the flood of the Hwang Ho River, in China resulted in more than 4 million people drowning when it suddenly overflowed its banks in a violent flood. The lethal nature of this flood was due to the unconfined nature of the Hwang Ho River valley. The Hwang Ho flowed over a flat plain as it moved toward its terminus. The level of the channel bed had also risen with time, resulting in a greater sensitivity of the area to flooding. When increased discharge caused water levels to rise, there was little to hold back the flood waters.
Flood Plains. Flood plains are the flat areas of deposition that are formed when a stream overflows its channels during periods of flooding.
Floods
and Flooding Cycles.
We do not think of floods as major threats to life and limb, in part because
we experience small floods each year. In 1996 over 700 people were killed, and
many thousands displaced when the Yangtze River, in China flooded. The Johnstown,
Pennsylvania flood in 1889 resulted in over 2,500 deaths. Today the risk of
death due to flooding is somewhat lower than in 1889 because the occurrence
of flooding of river systems can be predicted using an occurrence table, as
shown below. Flood frequency, or recurrence interval can be predicted on the
basis of the discharge of the river.
