natural disasters topic 8 (drainage basins & rivers)

As a stream system transitions from the collecting zone to the transporting zone, channels become wider and flatter with fewer tributaries.

Collecting/Transporting Transition Zone

The transporting zone is dominated by rivers that cross the plains and demonstrates a balance between erosional and depositional processes.

Transporting Zone

Photo by W. W. Little

Rivers

Rivers consist of channels that carry water and sediment from the collecting zone to the depositional zone.

Types of Rivers

There are two primary types of rivers, braided and meandering.

MeanderingBraided

Meandering Rivers

Meandering streams consist of a single, ribbon-like channel that weaves back and forth across its floodplain. These rivers are found where there is adequate discharge to carry all the available sediment, such as across broad plains.

River Components

The two major subdivisions of a river system are the channel and the floodplain, which are separated from each other by levees. Oxbow lakes and yazoo streams are also common features.

Modes of Sediment Transport

There are three ways in which sediment is transported by rivers, bedload (rolling, sliding, saltation), suspended load (floating), and dissolved load (individual ions).

Stream Velocity

Because of friction along the banks, flow velocity in a straight channel is highest near the surface in the middle of the stream.

Stream Velocity Along Bends

Curvature in a channel focuses stream velocity toward the outside of the bend, increasing rates of erosion along the outer bank and producing an asymmetrical channel profile.

Cut Bank Erosion

Erosion on the cut bank causes the entire channel to migrate toward the outside of the bend.

Stream Velocity & Meandering

Erosion along the outer bank of a channel bend causes the bend to migrate. This creates a meandering pattern, as the line of maximum flow velocity crosses back and forth across the channel from one bend to the next.

Helical Flow

Water moving through a river bend develops a corkscrew motion that is downward along the outer bank, further increasing erosion, and upward along the inner bank, resulting in deposition.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Point Bar Deposition

Deposition on the inner bank produces gently sloping sand layers that stack laterally as the channel migrates with outer bank erosion. These form bodies called point bars.

Photo by W. W. Little

Meander Cutoffs

As meander loops grow, they can eventually meet, creating a “shortcut” as the old loop is cut off from the main channel. The result is an oxbow lake.

Photo by W. W. Little

Flood Plains

Flood plains are flat areas adjacent to river channels that are covered with water during floods.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Braided Rivers

Braided streams consist of many shallow channels that bifurcate and rejoin. These rivers are found where there is more sediment available than can be carried by the available discharge, such as mountain foothills.

When a stream contains more sediment than can be carried by its discharge, it drops the excess and flows around it, forming interchannel bars.

Channel Bifurcation

Photo by W. W. Little

Floods

Floods occur when there is more water than the channel can contain. Floods can be of varying sizes, covering part or all of the flood plain.

July 1988

July 1993

Photo by W. W. Little

Photographer unknown

Flooding & Urbanization

Because of the amount of concrete and asphalt in urban areas, precipitation that would normally infiltrate through the surface and move to the water table is, instead, funneled to river channels, increasing the frequency and severity of flooding unless prevented by artificial measures.

Photo by W. W. Little

Flood Control

Primary flood control measures include building levees or walls to contain flow within the river, digging bypass channels to divert water around flood-prone areas, and constructing dams to create reservoirs for temporary water storage.

Photo by W. W. Little

Levees

Levees are used to contain flow to within or near the channel.

Photo by W. W. Little

Walls

Walls are constructed to contain flow to within the channel.

Photo by W. W. Little

Bypass Channels

Bypass channels are used to divert flow around flood-prone areas.

Reservoirs

Reservoirs are built to catch and contain water before it reaches the channel.

Base-level Fluctuation

Stream terraces are formed as:1. A Stream near base level creates a floodplain2. The land is uplifted or base-level falls3. The stream cuts to the lower base-level and carves a new floodplain.

Stream Terraces

The dispersal zone occurs where streams empty into lakes and oceans or onto a valley floor.

Dispersal Zone

When a stream enters a body of standing water, such as a lake or the ocean, velocity decreases and the sediment it has been transporting is dropped to form a delta.

Deltas

Deltas consist of two main components, distributary channels that form the main framework and splays which fill the space between distributaries.

Delta Formation

When a stream enters standing water, its velocity drops to almost zero and sediment is dropped in the middle of the channel forming a bar. The stream then bifurcates into smaller channels that move around the mid channel bar.

Channel Bifurcation

Photo by W. W. Little

Photo by W. W. Little

Photo by W. K. Hamblin

Flooding can breach the channel’s levees, allowing a smaller delta, called a splay, to develop between the main distributaries.

Splays

As a stream builds further into the basin, its slope decreases to near zero. The stream then will typically break through its margin at some point upstream and find a steeper path to the basin. The abandoned lobe then subsides and is reworked by marine processes.

Delta Evolution

Deltas can have a variety of shapes depending upon the relative dominance of river, wave, and tidal processes.

Delta Types

Alluvial fans form on flat valley floors at the mouths of deep mountain canyons as the stream gradient flattens and flow becomes unconfined.

Alluvial Fans

Bajadas consist of the coalescence of adjacent alluvial fans to form a blanket.

Bajadas

Photo by W. W. Little

When a stream empties into a body of water located at the base of a steep mountain front, it will often produce a feature that has characteristics of both deltas and alluvial fans.

Fan-Deltas