Riverscour Ecosystems of Eastern Unglaciated North America: A Review

Riverscour is an eclectic assemblage of highly biodiverse riparian natural communities that share characteristics with grasslands, savannas, glades, wetlands, and floodplains. We define "riverscour" as "open riparian habitats of rocky, stable-substrate (bedrock, boulder, cobble) zones, often along high-gradient streams, where periodic high-energy flows (water, ice, debris) and edaphic factors inhibit woody vegetation and promote persistent grassland-shrubland-open woodland-outcrop communities rich in conservative heliophytes." A key factor distinguishing riverscour from gravel and sand bars and other floodplain habitats is that these areas are underlain by more stable substrates, which resist structural reworking by floodwaters. Within Eastern Unglaciated North America, we mapped 1322 stream reaches totaling 2385.8 km containing riverscour. Given their small size, these communities support a disproportionately large number of rare, endemic, and undescribed species. For example, within a five-county area in Tennessee, riverscour makes up significantly less than 1% of the area but contributes at least 37 (25%) of the region's 150 state- and federally-listed vascular plant species. There are numerous threats to riverscour, the greatest being inundation caused by impoundment of rivers and associated downstream hydrologic alterations. Interruption of scouring processes associated with flooding and/or ice promotes succession toward larger woody species and away from open herbaceous/shrub-dominated vegetation. Other threats include invasive species, recreation pressure, and climate change. These threats, coupled with high biodiversity and historical losses, make protection and proper management of riverscour ecosystems especially important in conserving the native biodiversity of eastern North America.