The Floodplain

The floor of most stream valleys is relatively flat. This is because over time the stream moves back and forth across the valley floor in a process called lateral migration. In addition, periodic flooding causes sediments to move longitudinally and to be deposited on the valley floor near the channel. These two processes continually modify the floodplain.

Through time, the channel reworks the entire valley floor. As the channel migrates, it maintains the same average size and shape if conditions upstream remain constant and the channel stays in equilibrium.

Two very general types of floodplains may be defined:

• Hydrologic floodplain, the land adjacent to the baseflow channel residing below bankfull elevation. It is inundated about two years out of three. Not every stream corridor has a hydrologic floodplain.

• Topographic floodplain, the land adjacent to the channel including the hydrologic floodplain and other lands up to an elevation based on the elevation reached by a flood peak of a given frequency (for example, the 100-year floodplain).
  Professionals involved with flooding issues define the boundaries of a floodplain in terms of flood frequencies (probability of recurrence, in years). Thus, 100-year and 500-year floodplains are commonly used in the development of planning and regulation standards.

Flood Storage

The floodplain provides temporary storage space for floodwaters and sediment produced by the watershed. This attribute serves to add to the lag time of a flood--the time between the middle of the rainfall event and the runoff peak.

If a stream's capacity for moving water and sediment is diminished, or if the sediment loads produced from the watershed become too great for the stream to transport, flooding will occur more frequently and the valley floor will begin to fill. Valley filling results in the temporary storage of sediment produced by the watershed.

Landforms and Deposits

Many topographic features are formed on the floodplain by the lateral migration of the channel. These features result in varying soil and moisture conditions and provide a variety of habitat niches that support plant and animal diversity.

Floodplain landforms and deposits include:

• Meander scroll, a sediment formation marking former channel locations.
• Chute, a new channel formed across the base of a meander. As it grows in size, it carries more of the flow.
• Oxbow, a term used to describe the severed meander after a chute is formed.
• Clay plug, a soil deposit developed at the intersection of the oxbow and the new main channel.
• Oxbow lake, a body of water created after clay plugs the oxbow from the main channel.
• Natural levees, formations built up along the bank of some, generally low-gradient streams that flood. As sediment-laden water spills over the bank, the sudden loss of depth and velocity causes coarser-sized sediment to drop out of suspension and collect along the edge of the stream.
• Splays, delta-shaped deposits of coarser sediments that occur when a natural levee is breached. Natural levees and splays can prevent floodwaters from returning to the channel when floodwaters recede.
• Backswamps, a term used to describe floodplain wetlands formed by natural levees.