Effects of Aquatic Macrophyte Cover on Bed Roughness in the Shasta River, California

Aquatic macrophytes create seasonally variable changes in bed-form roughness as well as changing the effective surface roughness of the channel. Transects were sampled monthly for aquatic macrophyte coverage and depth at stream cross-sections on the Shasta River of northern California from March to October of 2007 to evaluate the effect of plant cover on roughness. An empirical relationship between measured 15-minute flow data at Montague Weir and manual discharge data collected at the Nelson Ranch was developed based on 2007 observations and tested against 2008 observations. A survey of water surface elevations taken longitudinally on the stream reach, the cross-section hydraulic geometry, and discharge estimated from the empirical model were used to calculate an apparent Manning's n roughness for each sample event. Manning's n was compared to measured cover of aquatic macrophytes in each transect. The relationship between plant cover and roughness changed over the growing season. Initial expansion of plant cover during the spring was coincident with stable or decreased roughness in all transects. This may be due to reduced exposure of coarse bed material. Post-expansion thickening of the plant patches increased roughness in the later months of the growing season. The relationship between cross-section area and flow changed with plant growth over the growing season. At the peak of aquatic macrophyte coverage, Manning's n was roughly twice the value during low-coverage periods. This implies that rating curves developed for stream reaches with significant aquatic macrophyte growth must be re-parameterized frequently to remain relevant, unless measures are taken to eliminate plant growth.