A floodplain is an area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge. Over centuries, rivers move laterally within their floodplains, as flood events occur. This back and forth movement, throughout the width of a floodplain, is a natural process. A healthy river ecosystem, including its plants and animals, is accustomed to pulses of nutrients and sediment as this process occurs. Too little or too much nutrients and sediment over time can impair the health of the ecosystem. The type of land cover in a river’s floodplain can greatly affect the rate of nutrient and sediment loss.
The best land cover from an ecosystem health perspective is forest or forest combined with native grasses. This type of land cover has a sediment and nutrient loss rate that is optimum for river ecosystem processes.
As part of StreamSweepers’ 2014 data collection, land cover information has been generated for the Rapidan, Robinson and other tributary rivers of the Rappahannock River watershed. Floodplains were delineated and land cover data was generated through use of the Virginia Department of Forestry’s online software, InFOREST. The following table contains land cover percentages for each of the major tributaries of the Rappahannock western portion of the Rappahannock River.
Note the low percentages of the Rapidan and Robinson River floodplain in forest land cover.
Land Cover Nutrient and Sediment Contributions from Floodplains
Again using the InFOREST online, StreamSweepers estimated nutrient (nitrogen and phosphorus) and sediment (soil) loss in the major Rappahannock River tributary floodplains. Results were generally similar to Robinson River findings. The land cover classification currently contributing the most nitrogen is hay (see image below).
The type of land cover currently contributing the most phosphorus in the Robinson River valley is also hay (see image below).
Finally, the type of land cover currently contributing the most sediment in the Robinson River floodplain is conventional tillage cropland (see image below).