DEFINITION OF RIVER SYSTEM

Stream and river valleys have been preferred

sites for human habitation for millions of years, and they

provide routes of easy access through rugged mountainous

terrain and also water for drinking, watering animals, and

for irrigation. Most of the world’s large river valleys are

located in structural or tectonic depressions such as rifts,

including the Nile, Amazon, Mississippi, Hudson, Niger,

Limpopo, Rhine, Indus, Ganges, Yenisei, Yangtze, Amur, and

Lena. The soils in river valleys are also some of the most fertile

that can be found, as they are replenished by yearly or

less frequent floods. The ancient Egyptians appreciated this,

as their entire culture developed in the Nile River valley and

revolved around the flooding cycles of the river. Rivers now

provide easy and relatively cheap transportation on barges,

and the river valleys are preferred routes for roads and railways

as they are relatively flat and easier to build on than

mountains. Many streams and rivers have also become polluted

as industry has dumped billions of gallons of chemical

waste into our nation’s waterways.

However, stream and rivers are dynamic environments.

Their banks are prone to erosion, and the rivers periodically

flood over their banks. During floods, rivers typically cover

their floodplains with several or more feet of water and drop

layers of silt and mud. This is part of a river’s normal cycle

and was relied upon by the ancient civilizations for replenish-

ing and fertilizing their fields. Now, since many floodplains

are industrialized or populated by residential neighborhoods,

the floods are no longer welcome and natural floods are

regarded as disasters. On average, floods kill a couple of hundred

people each year in the United States. Dikes and levees

have been built around many rivers in attempts to keep the

floodwaters out of towns. However, this tends to make the

flooding problem worse because it confines the river to a narrow

channel, and the waters rise more quickly and cannot

seep into the ground of the floodplain.

Streams are important geologic agents that are critical

for other earth systems. They carry most of the water from

the land to the sea, they transport billions of tons of sediment

to the beaches and oceans, and they erode and reshape the

land’s surface, forming deep valleys and floodplains and passing

through mountains.

Streams and rivers are very dynamic systems and constantly

change their patterns, the amount of water (discharge),

and sediment being transported in the system. Rivers

may transport orders of magnitude more water and sediment

in times of spring floods, as compared to low-flow times of

winter or drought. Since rivers are dynamic systems, and the

amount of water flowing through the channel changes, the

channel responds by changing its size and shape to accommodate

the extra flow. Five factors control how a river behaves:

(1) width and depth of channel, measured in meters (m); (2)

gradient, measured in meters per kilometer (m/km); (3) average

velocity, measured in meters per second (m/sec); (4) dis

charge, measured in cubic meters per second (m3/s); and (5)

load, measured as tons per cubic meter (metric tons/m3). All

these factors are continually interplaying to determine how

the river system behaves. As one factor, such as discharge,

changes, so do the others, expressed as:

Q = w × d × v

All factors vary across the stream, so they are expressed

as averages. If one term changes then all or one of the others

must change too. For example, with increased discharge, the

river erodes and widens and deepens its channel. The river

may also respond by increasing its sinuosity through the

development of meanders, effectively creating more space for

the water to flow in and occupy by adding length to the

river. The meanders may develop quickly during floods

because the increased stream velocity adds more energy to

the river system, and this can rapidly erode the cut banks

enhancing the meanders.

The amount of sediment load available to the river is

also independent of the river’s discharge, so different types of

river channels develop in response to different amounts of

sediment load availability. If the sediment load is low, rivers

tend to have simple channels, whereas braided stream and

river channels develop where the sediment load is greater

than the stream’s capacity to carry that load. If a large

amount of sediment is dumped into a river, it will respond by

straightening, thus increasing the gradient and velocity and

increasing its ability to remove the added sediment.

When rivers enter lakes or reservoirs along their path to

the sea, the velocity of the water will suddenly decrease. This

causes the sediment load of the stream or river to be dropped

as a delta on the lake bottom, and the river attempts in this

way to fill the entire lake with sediment. The river is effectively

attempting to regain its gradient by filling the lake,

then eroding the dam or ridge that created the lake in the first

place. When the water of the river flows over the dam, it does

so without its sediment load and therefore has greater erosive

power and can erode the dam more effectively.

Rivers carry a variety of materials as they make their

way to the sea. These materials range from minute dissolved

particles and pollutants to giant boulders moved only during

the most massive floods. The bed load consists of the coarse

particles that move along or close to the bottom of the

riverbed. Particles move more slowly than the stream, by

rolling or sliding. Saltation is the movement of a particle by

short intermittent jumps caused by the current lifting the particles.

Bed load typically constitutes 5–50 percent of the total

load carried by the river, with a greater proportion carried

during high-discharge floods. The suspended load consists of

the fine particles suspended in the river. These make many

rivers muddy, and they consist of silt and clay that moves at

the same velocity as the river. The suspended load generally

accounts for 50–90 percent of the total load carried by the

river. The dissolved load of a river consists of dissolved chemicals,

such as bicarbonate, calcium, sulfate, chloride, sodium,

magnesium, and potassium. The dissolved load tends to be

high in rivers fed by groundwater. Pollutants such as fertilizers

and pesticides from agriculture and industrial chemicals

also tend to be carried as dissolved load in rivers.

There is a wide range in the sizes and amounts of material

that can be transported by a river. The competence of a

stream is the size of particles a river can transport under a

given set of hydraulic conditions, measured in diameter of

largest bed load. A river’s capacity is the potential load it can

carry, measured in the amount (volume) of sediment passing

a given point in a set amount of time. The amount of material

carried by rivers depends on a number of factors. Climate

studies show erosion rates are greatest in climates between a

true desert and grasslands. Topography affects river load, as

rugged topography contributes more detritus, and some rocks

are more erodable. Human activity, such as farming, deforestation,

and urbanization, all strongly affect erosion rates

and river transport. Deforestation and farming greatly

increase erosion rates and supply more sediment to rivers,

increasing their loads. Urbanization has complex effects,

including deceased infiltration and decreased times between

rainfall events and floods.

See also BRAIDED STREAM; FLOODPLAIN; FLUVIAL;

MEANDER.

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