THE EFFECTS OF ROCK HARDNESS AND STREAM POWER ON CHANNEL WIDTH, IN ARROYO SECO, MONTEREY COUNTY, CALIFORNIA

Rock hardness and available stream power can both affect stream channel width. A study was conducted along Arroyo Seco to determine the relationship between stream power, rock hardness, and stream-channel width. Arroyo Seco is a bedrock stream in northeastern part of the tectonically active Santa Lucia Mountains. A Schmidt hammer was used to make 50 measurements of rock hardness at 8 different outcrops in the Arroyo Seco stream channel. Bankfull channel width was determined in the field at all sites where rock hardness was measured, with the exception of one where measurement was prevented by dense vegetation. At that locality, channel width was measured on a 1:24000 scale map with a digital planimeter. The maximum measured hardness for a given outcrop should be an accurate representation of the true rock hardness. However, this approach neglects the degree of rock fracturing, which affects rock competence. The average value of rock hardness includes measurements that reflect the presence of microfractures, and may be a better measure of the competence of the rock. The quantitative relationship between the average hardness of an outcrop and rock competence has not been established, but it is reasonable to assume that a lower average hardness corresponds to less competent rock. Data analysis shows a weak linear correspondence (R² = 0.35) between average rock hardness and stream channel width. The correlation of average rock hardness to channel width is much stronger than the correlation of maximum rock hardness to channel width, reinforcing the idea that average rock hardness is more representative of rock competence than maximum rock hardness. Channel slope and width are weakly correlated (R² = 0.34). Available stream power does not correlate well with width, but the product of stream power and average rock hardness plotted against stream width yield a stronger linear relationship (R² = 0.66). Although average rock hardness alone does not correlate with channel width, average rock hardness and stream power together correlate reasonably well with channel width. In other words, relatively narrow channels are formed in reaches of Arroyo Seco where average rock hardness and stream power are relatively high.