Habitat change in braided flood plains (Tagliamento, NE-Italy)

1. Relative changes and age distribution of habitats were investigated in the active channel of a bar-braided and an island-braided reach of the Tagliamento River (NE-Italy). Between September 1999 and January 2002, six habitat types were delineated with a differential Global Positioning System on five dates following floods of different magnitude. Overlay maps were employed to calculate age and relative change of habitats. We established exponential decay rates (k-values) for islands and major aquatic habitats. 2. Relative changes of all aquatic habitats combined were up to 82% between survey dates in the bar-braided flood plain, with a cumulative rate of 85% over the 2.5-year period. Relative habitat changes in the island-braided flood plain were lower with a cumulative change of almost 60% during the study period. In the bar-braided flood plain significant exponential decay relationships were established for channels, alluvial channels, backwaters, and ponds. 3. Half-lives were particularly short for backwaters and ponds. In the island-braided reach, significant relationships existed for channels and alluvial channels. The half-lives of channels and alluvial channels increased with the presence of vegetated islands. Relative habitat composition within the active corridor remained almost constant, supporting the applicability of the shifting mosaic steady state model to braided floodplain ecosystems. 4. Our results indicate that under natural conditions aquatic floodplain habitats can be highly dynamic over short time-scales. Even small water level fluctuations ('flow pulses') can lead to major habitat changes with important consequences for the fauna and flora.