ABSTRACT
Islands are familiar landforms surrounded by water. Fluvial islands, those in stream channels, are subject to change by water, especially overtopping floods, and most form and later are eroded by streamflow. The question arises, therefore, as to which processes result in fluvial islands rather than lower channel exposures such as bars. This question is largely answered by defining fluvial islands. Most importantly, fluvial islands are surrounded by channel. In perennial streams, they are higher than mean water level and persist long enough to have permanent vegetation. For intermittent and ephemeral streams, an island extends sufficiently above the channel to maintain well-defined banks, viability during most floods, and vegetation if moisture is adequate. Most fluvial islands are alluvial, formed of sediments deposited to flood-plain level, whereas others are results of incision or of rapid change in stream-channel position (avulsion). Some fluvial islands occur by erosion around a bedrock high, mass movement, glaciation, or eolian deposition. Regardless of the mode of formation, fluvial islands are channel features modified by streamflow. Thus, fluvial islands are unstable and transient over long periods, and although they rarely record long-term change, their shapes, sizes, and sediment may provide excellent information of recent stream processes and habitat.
