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Riparian Health - Riparian Areas and Water Storage

Last Updated: August 16, 2011

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Storing Water in a Riparian Area

The above photo is a good example of a healthy riparian area that is storing water. The willows which are spread throughout the valley, are evidence of a high water table.

A healthy riparian area includes much more than just the channel in which the water flows and the vegetation on the banks near it. Healthy riparian areas often spread out across floodplains and farther through the water table. Riparian areas have the ability to store and release water over a long period of time. This water is stored by the soil in the streambanks, floodplain, and substrate under the channel which acts as a sponge to retain water. This water can extend out beyond the banks, sometimes spreading to include entire valley bottoms. The water is held here until later in the year, when the stored water slowly seeps out of the “sponge.” As it seeps out, it is either percolated downward to recharge underground aquifers or moves back into the stream channel, thus extending the availability of surface water later into the year.

Bringing Water Storage Back into a Riparian Area

When a riparian area degrades, it loses its ability to store water, and water tables are lowered or lost altogether. A degraded riparian area recovers by capturing sediment, called "aggradation," which aids floodplain development and improves flood-water retention. This recovery is often most evident by the increase in riparian-wetland vegetation. For example, a change in species composition from upland species like sagebrush to riparian-wetland species like Nebraska Sedge in the floodplain and near the stream is a good indication that the riparian zone is once again capturing and storing water. Aerial photos are a great tool for documenting these changes near a channel.

Warning Signs

When a riparian area has degraded and downcut such as in this photo, the water table must drop with the stream level and can be lost.

Some warning signs that a riparian area may be starting to degrade and lose its ability to store water, which may lead to a decline in health or unraveling of riparian areas include:

Increase in upland vegetation in the riparian areas. Upland vegetation growing next to the stream and replacing riparian vegetation is a sign that less water is being stored in these areas.

Downcutting. Vertical instability or incision of a stream causes the floodplain to get flooded less often or for shorter periods. Much less water flows into the valley bottom aquifer through the banks than used to flow in through the floodplain surface. Water also drains faster out of the aquifer into the lowered stream. As the banks dry, the vegetation changes and fewer roots allow streambank soil to erode. Deep and fast floodwaters prevent the stream from capturing sediment (to build a new sponge) until the incision gets very wide.

Restoring water storage

After streams incise a little but before they get past a depth of no return, they may come back up with renewed balance in the sediment supply, more roughness from riparian vegetation or woody debris, or a lower gradient from restored or reformed meanders. When the stream floods its floodplain frequently (between 1 and 3 years on average), water from floods can again be stored under the floodplain. Streams that cut down too deeply lose their ability to reverse incision. High stress on the channel bed tends to increase erosion and incisions may become very deep. These channels widen because weakened banks allow rapid erosion. Eventualy, wide shallow flows in the bottom of the gully cannot transport their load like they could when they were narrow and deep. Then bars begin to form and vegetation can help them stabilize into a new smaller and lower floodplain. As aggradation continues, water storage generally increases.

Exceptions to the Rules

Not all channels are capable of widening and spreading water out into adjacent floodplains and streambanks. For channels that are steep, deeply entrenched and confined, such as Rosgen’s A1 channel type, the channel is incapable of widening and little water is stored within the banks. Also, in these channels, vegetation is not needed for bank stabilization, and the landform dictates much about channel shape and water and sediment movement.

References

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

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