Rubble (Boulder, Cobble) Streambed
Streambeds formed from rubble (boulder and cobble) are found exclusively along mid- and upper-reaches of streams in the Cumberland Plateau, Cumberland Mountains, higher ridges of the Ridge and Valley, and Blue Ridge Mountains. These are mostly associated with first- and second-order streams. These communities are largely devoid of vascular plant vegetation with the exception of riverweed (Podostemum ceratophyllum) and aquatic bryophytes and some algae. In many examples, water does not cover the rocks of the streambed except during periods of heavy rain. In a few streams, such as the Caney Fork River, water may run completely underground for much of the drier parts of the summer and fall resulting in "dry" boulder-strewn stream channels. In these cases water flows beneath the masses of boulders and emerges farther down in the stream system in lower parts of the gorge.