Southeastern Riverscour Communities

What on earth is a riverscour? 

Riverscour communities are open, rocky (bedrock, cobble, boulder, or mix) riparian communities dominated by shrubs, herbs, and grasses. They differ from sandbars and gravel bars in being highly physically stable over long periods of time, meaning they don’t shift or disappear frequently. Due to their stability and the fact that they are kept open by periodic flash floods, they often support relictual associations of species that are often rare or disjunct from other regions. Those in the Eastern Deciduous Forest region of the southeastern US are particularly interesting because by their very nature they are island-like in distribution, both within a given stream system and obviously between two separate streams. Most southeastern cobble bars are entrenched in deep gorges in mountainous regions (Fig 1) and are sandwiched between large tracts of dense forests and boulder-strewn rivers. 

Figure 1
Fig 1: Riverscour cobble bars abound in deeply entrenched rivers of the Cumberland Plateau of TN, KY, and AL.
Figure 2
Fig. 2. The federally-threatened Virginia spiraea (Spiraea virginiana) on a riverscour in Scioto Co., OH.  Photo by Andrew L. Gibson.

Despite being embedded in densely forested landscapes, riverscour communities are often dominated by species more frequently found in prairies, pine savannas, and other open habitats. There are several federally-listed rare species restricted to riverscour communities, including Virginia spiraea (Spiraea virginiana, Fig. 2) and Cumberland rosemary (Conradina verticillata). Talk about out of place!

Recent work shows that there are at least 10 new species associated with southeastern riverscour. What’s more is that each river system often has its own set of new species. This work raises important questions about speciation in these specialized habitats and eventually will lead to collaborations with other scientists to understand the deep history of these communities. Finally, this work is part of a larger multi-year effort that the Estes lab is working on to systematically study the various riverscour systems. Four graduate students are currently working on riverscour systems in TN, KY, and AL.

Project Goals

  • Documenting the flora of southeastern riverscour
  • Systematically searching for undescribed species
  • Collecting plot data to better understand ecology
  • Compare the flora and vegetation of the varoius riverscour types
  • Document the phytogeographic patterns of riverscour plants
  • Partner with phylogeographers to study the genetic structure of riverscour species
  • Use these data to better understand the evolution and history of riverscour communities

Why focus on riverscours? 

Riverscour zones are primarily found in rugged and remote areas. In fact many are located in some of the most remote places in the Southeast. Because of their remoteness, in conjunction with their extreme ruggedness and relative inaccessibility (see the image below), they have been underexplored. This coupled with the fact that they are extreme communities known to support high-levels of endemic and rare species makes them some of the most likely places to find new botanical discoveries in eastern North America.

Getting to the riverscours is only half the battle

Given that many of the riverscour systems are located in remote gorges, it can be very challenging and dangerous to study them. Many are lined with car- and room-sized boulders, and some are walled-in by vertical cliffs. In a given stream the river repeatedly alternates from deep pools (the one pictured is >60 ft deep) to boulder-choked rapids. During one survey of the Obed River Gorge in 2007, we encountered 11 copperheads on cobblebars in a single day within a 3-mile stretch of river. Surveying in spring is difficult because of high waters and white-water conditions requiring experienced kayaking skills. Surveying in summer and fall is mostly only possible by a combination of rock-hopping, wading, swimming, and using small light-weight rafts or floats.

Types of riverscour communities

Riverscours vary widely according to rock type, substrate structure, vegetation, and flow rate of the associated stream. Riverscours are associated with limestone, dolomite, sandstone, granite, and other types. The substrate may consist of exposed bedrock that lies horizontally (forming riverscour glades) or may be strongly tilted in metamorphosed examples (e.g., Ocoee River, TN). The rock may also consist of unconsolidated materials, including a mixture of gravel, cobble, sand, and boulder. In addition to studying floristic differences among different river systems, we also aim to study differences within a river system based on these different habitats.