What is a Floodplain?
Schmudde (1968) provides three definitions of floodplain. Topographically, it is flat and lies adjacent to a stream. Geomorphically, it is a land form composed primarily of unconsolidated depositional material, or sediments, derived from the stream. Hydrologically, it is a land form subject to periodic flooding by the stream.
Floodplains develop over time as the result of flood inundations. The water moving over a floodplain travels at a lower velocity than the channel flow, and as flow velocity decreases, sediment is deposited. Over time, these deposits of nutrient-rich sediment are built up in layers. The sediment also brings nutrients to the riparian vegetation which is growing upon it.
Function of Floodplains
Schmudde provides a good summary of the functional purpose of the floodplain: “Thus, the floodplain is seen as an integral part of the stream system and the adjustment mechanism needed to meet the requirement of discharge and load imposed by the basin it serves.”
When evaluating a riparian area, it is important to determine if frequent floodflows are capable of spreading out on a floodplain adjacent to the stream thereby providing for energy dissipation, sediment deposition, and periodic flooding of vegetation. Stream systems that are not highly confined generally support a floodplain landform that is flat and adjacent to the stream. However, if the channel is downcut and floodflows can no longer access the floodplain, it no longer provides those important hydrologic functions.
The floodplain provides the additional capacity for the stream system to transport and store water and sediment. The magnitude and significance of the additional capacity depends on the spatial extent of the floodplain along with basin and stream system characteristics. Vegetation often is an important player in the efficiency and longevity of floodplain function. Periodic flooding of the floodplain is often necessary to promote and sustain riparian vegetation and therefore is a key factor in determining the functional condition of the riparian system.
Understanding Bankfull
The floodplain is functional if it is normally connected to the stream at the bankfull discharge point of the channel. Wolman and Leopold (1957) suggest there is an annual flood that normally reaches the floodplain every year or so. Gebhardt, et al., (1989) call this area of inundation the “active floodplain” to distinguish floodplain activity form floodplain inactivity. Thus, an active floodplain would see some inundation every year or so, and the spatial extent of the inundation would increase over the floodplain as the magnitude of the flood increases. In other words, the larger the flood, the more floodplain will be used. The loss or reduction in the ability of the floodplain to dissipate energy and transport water and sediment are key factors in contributing to loss of functionality.
Evaluation of bankfull discharge is key in determining whether the topographic floodplain feature is connected to the stream. Water enters the floodplain when flows begin to exceed bankfull discharge. Bankfull discharge is significant for riparian resource management in that it represents a measure of interaction between the stream and it's adjacent valley bottom; thus, it strongly influences the geomorphic and biological characteristics of the riparian environment. Bankfull discharge on the majority of streams in the world has a recurrence interval between one and three years; one and a half years is considered a reasonable average. Hence, the floodplain will be accessed in relatively frequent events.
Bankfull stage, or elevation of water surface, can be identified in the field through several observable features:
- Top of point bar,
- Changes in vegetation,
- Topographic break in slope,
- Change in size, staining, or color of substrate material, and
- Change in the nature and amount of debris deposits (Leopold, 1994)
It is best to have more than one indicator present, as just one -- for example, vegetation -- can be indicative of more short-term events than the channel-forming flows.
Bankfull discharge for a channel may be determined through the use of one or more surveyed cross-sections and a number of hydraulic models. These are best done by trained professionals who use the data that are collected from them. Your local Extension Agent can help you find a trained person.
Warning Signs
Some warning signs that the floodplain is not being inundated, which may be indicative of declining health or “unraveling” of riparian areas include:
Oversized Channels – Channels have widened to the point that the floodplain is not accessed fairly frequently.
Downcut/Incised Channels – Channels that have downcut are incapable of accessing their floodplains and will begin the process of establishing a new floodplain at it's lower levels, if possible.
Exceptions to the Rules
There are situations in which floods do not reach the floodplain due to flows being regulated -- either through reservoirs or diversions. The regulation of flow keeps the stream from having to dissipate energies on a floodplain regularly.
Related Pages
Riparian Health - Evaluating the Health of Riparian Areas - An Overview
Riparian Health - Understanding the Role of Beavers in Riparian Areas
Riparian Health - Understanding if the Channel is in Balance with the Landscape
Riparian Health - Riparian Areas and Water Storage
Riparian Health - Understanding How Uplands Contribute to Riparian Health
Riparian Health - Understanding Riparian Vegetation Age-Class and its Role in Health
Riparian Health - Understanding Species Diversity
Riparian Health - Understanding the Relationship between Vegetation and Soil Moisture Characteristics
Riparian Health - Understanding Root Masses and Bank Stability
Riparian Health - Understanding Plant Vigor
Riparian Health - Understanding if you have Adequate Vegetation
Riparian Health - Understanding the Role of Large Woody Material in Riparian Areas
Riparian Health - Understanding a Channels Ability to Dissipate Energy
Riparian Health - Point Bars
Riparian Health - Understanding Lateral Stability in Riparian Areas
Riparian Health - Understanding Vertical Stability in Riparian Areas
Riparian Health - Understanding if the Channel is in Balance with the Soil and Water Being Supplied
References
Gebhardt, K., C. Bohn, S. Jensen, W.S. Platts. 1989. Use of hydrology in riparian classification. In Practical Approaches to Riparian Resource Management – An Educational Workshop. Bureau of Land Management. Billings, MT. pp. 53-59.
Leopold, L.B. 1994. A View of the River. Harvard University Press, Cambridge, Massachusetts. 298 pp.
Schmudde, T.H. 1968. Flood plain. In The Encyclopedia of Geomorphology, edited by Fairbridge, R.W. Reinhold Book Corporation, NY. Pp. 359-362
Surber, Gene and Bob Ehrhart. 1998. Stream and Riparian Area Management. Montana State University Cooperative Extension Service. Information also available at http://www.animalrangeextension.montana.edu/riparianmgt/index.htm
USDI Bureau of Land Management. 1998. Riparian Area Management: A User Guide to Assessing Proper Functioning Condition and the Supporting Science for Lotic Areas. Technical Reference TR 1737-15. 124 pp. More Information available at: http://www.blm.gov/or/programs/nrst/index.php
Wolman, M.G. and L.B. Leopold, 1957. River floodplains: some observations on their formation. USGS Professional Paper 282-C.
Zaimes, G.N. (editor). Understanding Arizona’s Riparian Areas. Univeristy of Arizona Cooperative Extension, Publication #az1432. Available online at: http://ag.arizona.edu/extension/riparian/