The Persistence of Potential Refugia Mapped from Gravel Tracers

Floods disturb aquatic habitats. On an event basis, flood characteristics control the spatial extent and depth of streambed disturbance for a given river and set limits to the amount of channel refugia for biota. The aim of this research is to quantify the area of potential refugia that persists over a long flood series and therefore affects many generations of aquatic populations. Field observations were collected in Carnation Creek, a small gravel-bed river located on the west coast of Vancouver Island, Canada. Streambed disturbance was documented by monitoring the three-dimensional positions of about 2500 magnetically tagged gravels over 277 floods. Tracer movement and burial observations were used to produce cellular maps of the frequency of bed disturbance within a GIS. The streambed exhibits different frequencies of disturbance as expected. The most active areas make up about 1% of the streambed and tend to be located near the channel thalweg. Undisturbed areas constitute more than 25% of the bed, and provide distinct areas of longer-term refugia that persist over the range of flood magnitudes observed. In addition to validating a key aspect of partial sediment transport, the results suggest that the natural variability of floods facilitates diverse aquatic communities by ensuring the availability of channel refugia over time.