Scaling the effects of riparian vegetation on cross-sectional characteristics of ephemeral mountain streams—a case study of Nahal Oren, Mt. Carmel, Israel

To assess the effect of trees and shrubs on stream structure on different spatial scales, we tested the relationships between their percentage coverage in relation to width, depth and stream slope. This research was conducted during the winter and spring of 2004 at Nahal Oren, Israel, an ephemeral mountainous stream. Our hypothesis, that in the presence of high vegetation cover a deep and narrow stream will form, was based on other studies showing that vegetation stabilizes stream banks. Additionally, we hypothesized that shrub and woody vegetation would have similar effects on stream morphology, and that the spatial scales of their effects will be limited. Four 50 m segments were chosen in the middle part of Nahal Oren, each being characterized by a different vegetative structure. In each segment six cross-sections were preformed 10 m apart as well as a longitudinal profile. Additionally, at three of the cross-section sites vegetation transects were conducted at mid-bank up stream of the cross-sections. Average width/depth ratios at the different sites varied from 5.5 to 9.5 and were not correlated with channel slope. Regression analysis indicated that tree coverage is positively correlated with higher width/depth ratios, at a distance of 1 m above the cross-section. The average shrub coverage at 5 m was negatively correlated with width/depth ratios. We suggest that shrubs and trees may have a similar function in stabilizing banks via the root system, but the above-ground structure of these vegetative types has different effects. Possibly the dense foliage of the shrubs reduces water velocities, thus further increasing bank stability.