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 influences on 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 a river 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 supplied from 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 the river 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 and carry 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 large material. 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, and clay. 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. The steepness 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 the lowest 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 not enough 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 with the 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. Other rivers, such as those in desert regions, receive water only during brief, intense storms that may cause a flash flood. 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 water found 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 river channel may have lots of water flowing though it during the rainy season but be dry as a bone in the late summer.

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 the intensity 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 and energy 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 sooner than 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 channel and 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 for short periods, the water may not sort the sediment and may deposit the particles in a mixture ranging in size from coarse to very fine.

River patterns, or general shapes, depend on the geologic zone and the climate of the location. There are four river patterns: meandering, braided, anastomosing, and straight. A meandering pattern follows a winding, turning course. A braided pattern has connected channels that resemble a hair braid. An anastomosing river pattern combines 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 in the middle and mouth zones of most rivers.  

The Mississippi River is a classic example of a meandering river that has looping bends of different sizes along its valley. Each bend is the result of sediment depositing on the inside of the bend. As sediment deposits gradually build up, a point bar forms on the inside of the bend. The point bar pushes the river flow against the outside bank of the bend, eroding the bank opposite the point bar. Eventually the bend becomes so sharp that the river bypasses it, cutting a straighter path. The arc of the bend is left behind as the river moves past. The arc may form an oxbow lake (also called a billabong), a pool of water enclosed by the arc and riverbank. A meandering river’s bed is usually covered with sand, while the floodplain is filled with silt and clay.

Braided rivers look completely different from meandering rivers. They have many channels that are constantly changing position because of frequent changes in flow rate and sediment supply. The channels of a braided river—such as portions of the Platte River that flow through Nebraska—change course frequently, so the river’s water may cover the entire floodplain on a regular basis. The sediments of braided rivers are usually gravel and cobbles. Sometimes a meandering river may change into a braided river in the middle zone if the supply of sediment increases as a result of farming or grazing activities in the watershed.

Anastomosing rivers combine the bends of meandering rivers with the multiple channels of braided rivers. The sediments are typically sand, silt, and clay. Oxbow lakes may be rare. The Amazon River is an example of an anastomosing river.

Straight rivers are not common. They are typically located in canyons in mountainous areas or exist as the result of engineering structures that force a river into a straight course. Portions of both the Columbia River (between Washington and Oregon) and the Colorado River (in the southwestern United States) flow straight through canyons.