Image analysis of aerial photography to quantify changes in channel morphology and instream habitat following placer mining in interior Alaska

Abstract

SUMMARY1. ‘Placer’ mining for alluvial deposits of gold in a number of stream systems in interior Alaska represents a major disturbance to the stream bed and affects habitat for biotic communities.2. The potential of analysing aerial photographs to map changes in channel habitat and morphology within gravel‐bed rivers is outlined with reference to the impact and recovery of Faith Creek, a second‐order stream with a history of placer mining.3. A strong correlation between the reflectance of the channel bed and water depth is necessary to use the technique succesfully, together with a knowledge of the effects of ‘broken’water on the spectral characteristics of rivers.4. Image analysis demonstrated that a wide range of water depths and instream mesoscale habitats existed prior to mining. During mining, the stream was confined to a channellized reach with negligible deep water or habitat diversity.5. Since mining ceased the stream has abandoned its channellized course and formed a new channel with few deep pools. It is suggested that geomorphological recovery and associated habitat recovery takes a number of large flood events and is likely to require more than 10 years.