QUANTIFYING FLOODPLAIN DYNAMICS

An active floodplain provides a diverse range of aquatic habitat suitable for multiple species and life stages of Pacific salmon. The magnitude of floodplain dynamics can be characterized using geomorphological channel patterns (straight, meandering, island braided, and braided), where the intermediate categories (meandering and island braided) tend to provide the highest habitat diversity. Although this qualitative classification is easily understood and provides a useful concept for discussing floodplain dynamics, it is ambiguous, relying largely on the subjective interpretation of patterns on the floodplain. We present and compare several quantitative methods of interpreting floodplain patterns based on simple models of floodplain dynamics. These metrics provides a more objective method for examining the links between floodplain dynamics, and habitat and species diversity. All of the metrics are based on the age class distribution of vegetation in the floodplain. Age class information is calculated by digitizing aerial photographs, delineating the floodplain, and estimating the age class of vegetation at points on a grid placed over the floodplain. The most active floodplains are dominated by points without vegetation or young vegetation, while more stable floodplains are characterized by large areas of late successional forest. The channel pattern metrics are derived from two approaches. In the first, we borrow the return interval concept from the forest fire literature. By quantifying the amount of floodplain in each age class and assuming a constant probability of disturbance across space and time, we can estimate the disturbance return interval. In the second approach we use multiple aerial photographs of the same site taken at different times to estimate the probability of making a transition from one vegetation state to another. We found that the metrics based on the return interval were more stable than the transition probabilities. In addition they were much easier to derive since they required only one aerial photograph that did not need to be orthorectified. We present preliminary results comparing the new quantitative metrics with the traditional qualitative descriptions, and discuss ongoing field work aimed at linking flood plain dynamics to habitat and species diversity.