Meandering reduces the influence of stream orientation on riparian shade

Abstract

AbstractRestoring riparian shade helps maintain healthy stream ecosystems and computer models are helpful in guiding restoration. The effects of meandering and orientation on shade to direct and diffuse lighting were investigated using a simplified model of riparian vegetation. Previous studies have shown that straight streams oriented east–west (EW) experience higher direct lighting than those oriented north–south (NS). Meandering decreased reach‐averaged direct lighting in EW valley streams, increased lighting in NS valley streams but had little effect on diffuse lighting. When meander amplitude equalled 50% of meander wavelength lighting of streams in EW and NS‐aligned valleys was similar. In EW valleys, lighting was highest at stream bends and a strategy to increase shade quickly would be to prioritize planting tall vegetation on bends. In NS valleys, lighting was lowest at bends suggesting the opposite strategy. Shade exceeding 70%, a criterion for preventing nuisance aquatic plant growths and stressfully high water temperatures, occurs in ‘typical’ meandering streams on cloudless, mid‐summer days at mid‐latitudes once the ratio of tree height to stream width H/W exceeds about 2. The model over‐simplifies riparian vegetation and the effects of canopy shape and overhang merit further investigation.