Measuring stream shade

Despite the ecological importance of the exposure of streams to sunlight, quantitative measurements of stream shade have seldom been reported, apparently because of difficulties of measurement. We have developed methods to characterize shade over streams using a canopy analyzer, an instrument based on fisheye lens optics and designed for measuring indices of plant canopy structure. Point-to-point variability of lighting and average shade characteristics of a stream reach can be rapidly measured with this instrument. Fisheye lens photographs, which permit qualitative interpretation of stream shade, are a useful adjunct to canopy analyzer measurements. The significant advantages of the canopy analyzer method over alternatives such as visual survey, or long-term logging with photoelectric or photochemical sensors, are emphasised.