Effects of channelization and levee construction on western Tennessee floodplain forest structure, composition, and function

Background/Question/Methods

In the southeastern U.S., as in many areas, changes in land use and land cover in riparian zones have been made possible by the implementation of stream management practices designed to expedite or regulate water flow and alter the flood regime. With greater pressure on the utilization of floodplain systems, there exists a global effort to understand floodplain functions so land stewards may assess effects of anthropogenic manipulations and success of restoration efforts. In an effort to examine channelization effects on western Tennessee floodplains, we selected six river locations, including two non-channelized reaches, two channelized and leveed reaches, and two channelized but non-leveed reaches. Data on vegetation composition and structure, soil and leaf nutrient pools, soil redox potential, and surface water hydrology were compared among channelization treatments and floodplain microtopography (depression and nondepressional sites).

Results/Conclusions

Hydrology was significantly affected by channelization treatments, especially streams with levees. The disconnected floodplains were drier, maintained higher nutrient pools, and were more productive than floodplains still connected to channel hydrology. Land use and flooding stress are suggested as causes of this pattern. Channel and floodplain hydrology were most strongly connected for unchannelized streams. Unchannelized streams were varied in soil redox potential, water table, and nutrient pools. Vegetation composition reflects both historical regimes and disturbances, and thus complex relationships to channel modifications. Results suggest both the subsidy (i.e., nutrient inputs) and the stress of flood events have been altered by anthropogenic activities, but these alterations were greatest in channelized systems compared to unchannelized systems. Both channelized and unchannelized streams (with channelized tributaries) suffer from high sedimentation inputs, adding additional stress during flood events, resulting in decreased decomposition rates, nutrient cycling, and productivity. Over time, deposition of the sediment within the floodplain system can bury forests and plug channels, severely altering the biotic composition, physical structure, and hydrology of floodplains.