Geomorphological Processes and Human Impacts in River Basins (Proceedings of the International Conference held at Solsona, Catalonia, Spain, May 2004). IAHS Publ. 299, 2005, 231-242.

1 Department of Geography and Environmental Studies, University of Haifa, Haifa 31905, Israel

2 Department of Natural Resources and Environmental Management, University of Haifa, Haifa 31905, Israel

Abstract River adjustment to an artificial disturbance is examined in the ephemeral mountainous stream—Nahal Oren, Mt. Carmel, Israel. We compare sedimentological and morphological characteristics of two sections, 50 and 500 m downstream of an artificial pond. Our results indicate that the deprivation of bed load floodwater in the upper section generates high calculated entrainment potentials as indicated by the large differences between measured and calculated D₅₀ which exceed 80%. Each flow produces an effective flow (Q > 0.125 m³ s^-1) of between 17 and 58% of the flood duration to entrain pebbles of the D₅₀ size (50 mm). In a large flow of 12 m³ s^-1 (recurrence interval of 10 years), resulting bank erosion, in the upper section exceeded
4 tons per 20 m long section, whereas during the period 1999–2004 only minor degradation of <40 cm occurred at the lower section. The eroded material from the upper reach is loosely deposited, transported downstream and selectively deposited in the step-pool section. The impact of the pond on the channel at the upper section disappears 500 m downstream, where the site shows relative stability according to the lower calculated entrainment potential, the step-pool morphology of the reach and the higher ratio of cluster arrangements.

Key words bedforms; entrainment potential; Mediterranean streams; river adjustment