River - geography.

« In very large rivers, the water comes from rain that may have fallen as far as 6,000 km (4,000 mi) away.

During the journey through rills and streams, the water’s flowmay erode and deposit sediment in the river’s channel and on its floodplain ( see Erosion; Deposit).

The biggest rivers usually carry the largest amount of sediment.

Yet some of the largest rivers may carry very little sediment because the watershed may not have a lot of sediment.

A river carries the most sediment when the flow is thehighest.

When a river experiences high flows, it fills in (floods) the floodplain, a flat region of a valley surrounding the river channel.

As the water first reaches thefloodplain, it may erode the sediment on the floodplain.

As the flood drains from the floodplain, slower-moving water may deposit sediment onto the floodplain, replacingsome of the sediment lost. III FACTORS THAT SHAPE A RIVER River: From Source to Mouth© Microsoft Corporation.

All Rights Reserved. The combination of erosion and sedimentation in a river’s channel and on its floodplain works to produce the characteristic features of that river.

The three major influenceson patterns of erosion and deposition are geology, the type of sediment that is present, and the amount of water available.

From the perspective of geology, generally ariver travels through three zones from its headwaters at the top of the watershed to its mouth.

The headwater zone in the mountains or hills is where sediment is suppliedfrom hillsides and transported down steep channels with narrow floodplains.

In these narrow, steep canyons, the bed of the river may be covered with large boulders as theriver passes through many rapids.

When the mountains give way to the plains, the steepness of the river channel will decrease from as high as tens of meters per kilometer,a grade of 1 to 10 percent, to less than 1 meter per kilometer.

In this middle zone, although the amount of water may increase, the ability of the river to carve into rock andcarry sediment decreases because the river channel is less steep.

As the flow decreases, so does the power of the river, and the river loses its ability to transport largematerial.

Gradually, the sediment in the river decreases in size from boulders (larger than 256 mm/10 in in diameter) to cobbles (between 64 and 256 mm/ 2.5 and 10 in)to gravel (between 2 and 64 mm/0.08 and 2.5 in).

Eventually, as the steepness continues to decrease, the sediment becomes very fine, consisting mostly of sand, silt, andclay.

As the river changes in this middle section, the floodplain widens.

The third zone of a river is the zone influenced by the ocean or lake where the river ends.

Thesteepness of the river channel in this zone is usually less than 10 centimeters per kilometer, and the sediment is very fine.

If enough sediment settles out of the water in thelowest section of the flow, a river may form a delta.

A delta differs from a floodplain because in a delta the river splits into many new channels called distributaries.

If notenough sediment settles out to form a delta, the river may meet the sea in an estuary.

An estuary is usually a wide channel where the fresh water from the river mixes withthe salty seawater. River features are also affected by the flow rate and the size and duration of floods.

Some rivers receive rainfall almost every day in at least part of their watershed.

Otherrivers, such as those in desert regions, receive water only during brief, intense storms that may cause a flash flood ( see Flood Control).

The melting of snow and glaciers in the spring is a source of water for many rivers.

If a river flows year-round, the river is called a perennial river.

Usually a slow, steady inflow from groundwater, or waterfound underground, provides some of the water of a perennial river.

If a river flows only during part of the year, the river is called an ephemeral river.

An ephemeral riverchannel may have lots of water flowing though it during the rainy season but be dry as a bone in the late summer. Delta PatternsA delta is a triangular or fan-shaped deposit of sediment that forms at a river’s mouth where that river flows into an ocean, lake, orslower river.

As a river enters another body of water, its current slows and the sediment carried by the current is deposited.

If oceancurrents are strong enough, the sediment may be swept away, preventing a large delta from forming.© Microsoft Corporation.

All Rights Reserved. In the headwater zone of rivers, floods typically last a short time (less than one day) and are very powerful.

In the middle zone the duration of floods increases, but theintensity decreases because the area of the floodplain is larger.

At the mouth, or delta section of a river, floods can last for several months. The water flowing in a perennial river may do a great deal of work, eroding and depositing sediment in the channel and floodplain.

A perennial flow has enough time andenergy to separate the sediment by size.

The water moves coarser particles together in areas of the river where the water flows very fast.

It deposits these particles soonerthan finer particles, which are lighter and can stay suspended in the slower, less powerful flows.

In perennial streams, slower flows that occur within the floodplain area(they are slower because the land is not steep here) deposit the finer particles on the floodplain. In contrast to a perennial river, an ephemeral river may flow for only a few days.

Therefore, for most of the year, additional processes may affect the features of the channeland floodplain.

These processes include the action of the wind, the burrowing of animals, the growth of vegetation, and the activity of humans.

When flow occurs only forshort periods, the water may not sort the sediment and may deposit the particles in a mixture ranging in size from coarse to very fine. IV RIVER PATTERNS River patterns, or general shapes, depend on the geologic zone and the climate of the location.

There are four river patterns: meandering, braided, anastomosing, andstraight.

A meandering pattern follows a winding, turning course.

A braided pattern has connected channels that resemble a hair braid.

An anastomosing river patterncombines features of the meandering and braided patterns.

Some river patterns are simply straight channels.

Meandering and braided are the most common patterns.Braided and straight patterns are usually located in the mountains or hills below the headwater zone of rivers, while meandering and anastomosing patterns are located inthe middle and mouth zones of most rivers.. »