Steambank stability and cattle grazing in southwestern Montana

Cattle have impacts on riparian communities by grazing and trampling. Reevaluation of management practices indicated that implementing rest-rotation grazing management and limiting cattle use of riparian vegetation will reduce animal impacts. Rest-rotation and light grazing may improve plant vigor, but little information is available on how well either practice controls trampling damage on banks. A 4-year grazing study in southwestern Montana indicated both streamflow and cattle use were highly correlated with the degree of change in stream channel profile. The greatest streambank change occurred during periods of high streamflow (positive correlation) and low cattle use (negative correlation). However, further statistical analysis of the data indicated that streamflow itself was not a major factor in bank erosion. Although not significant in all years, the decline in channel change appeared related to the seasonal trend in soil moisture. As streambank moisture levels declined, the extent of channel alteration also declined. Channel profile changes in paddocks grazed after early August when banks had dried were not significantly different (P<0.05) from those in an ungrazed paddock. The interval between pre- and post-grazing measurements that was negatively related to channel change explained about 40% (r2=0.389) of the variation in channel profiles over the 4 years of study. However, examination of change in individual channel transects showed that measurement interval probably represented a seasonal or time-of-grazing effect. Streambank alteration may result from a combination of high soil moisture levels, high streamflow, and cattle use. Cattle use alone did not explain the degree of change in channel profile.