As riverine water levels drop following spring floods, patches of bare sand, mud, gravel, and cobbles are exposed. The riverine mudflat community is best developed within the floodplains of the state's largest, low gradient rivers, especially in central and southwestern Wisconsin. Soil development on the flats and bars is minimal, owing to the frequent flood disturbance. During the growing season these areas are colonized by an assemblage of herbs, and sometimes shrubs and saplings. The mudflats and beaches are highly variable in cover, being basically unvegetated in late spring/early summer transitioning to sparsely covered and eventually to locally dense stands of graminoids and forbs by late summer. Usually the vegetation is of short stature.
Significant floods affect this community annually, which may be accompanied by erosive scouring, sediment deposition, and sometimes (though not always) by shifts in the locations of the bars, mudflats and channels. Colonizing plants tend to be annuals, short-lived perennials, or perennials with light, wind, or water dispersed propagules adapted to quickly colonizing unvegetated substrates.
Plants that become established on these newly exposed, somewhat ephemeral habitats, include sedges, grasses, and a few woody species such as sandbar willow (Salix interior) and cottonwood (Populus deltoides). Short graminoids are initially prevalent, such as some of the "flat sedges", for example (Cyperus odoratus and C. squarrosus), spike-rushes (Eleocharis acicularis, E. obtusa, E. palustris), creeping love grass (Eragrostis hypnoides), tufted love grass (E. pectinacea), autumn sedge (Fimbristylis autumnalis), and small-flowered hemicarpha (Lipocarpha micrantha). Other native herbs associated with this assemblage are water star-grass (Lindernia dubia), marsh purslane (Ludwigia palustris), and moist bank pimpernel (Lindernia dubia).
In common with other high energy and frequently disturbed environments, such as the beaches and dunes along the Great Lakes, some opportunistic weedy species are also characteristic of riverine mudflats and beaches. However, as the slate is erased virtually every year, these tend not to be problems except in cases where the flood regime has been altered in a way that favors the weeds and development of a weed-dominated community. Examples include green carpetweed (Mollugo verticillata), black mustard (Brassica nigra), winged pigweed (Cycloloma atriplicifolia), and Russian thistle (Salsola tragus).
