The equivalent of many decades of natural or even agricultural erosion may take place during a single year from areas cleared for construction. Areas undergoing rapid development near Baltimore, Maryland, and Washington, D.C., lie on the Coastal Plain and Piedmont, with slopes generally of 1‐10% but sometimes of 20% and more. Soil is deep, and the annual precipitation of 42 inches (1100 mm) is evenly distributed, with high summer intensities. Average sediment yield is 200–500 t/mi2/yr (80–200 t/km2/yr), with predominantly wooded watersheds supplying sometimes even less. Intensive farming 50–80 years ago caused yields up to 1000 t/mi2/yr (400 t/km2/yr), but such high yields are no longer present owing to the continuous decline of farm acreage in the metropolitan periphery. Sediment concentrations from areas undergoing construction ranged from 3000 to over 150,000 ppm, whereas in natural or agricultural catchments the highest comparative concentration was 2000 ppm. In terms of annual values, yields from construction areas range from several thousand to a maximum of 140,000 t/mi2/yr (i.e., up to 55,000 t/km2/yr) from a small area. Total yield declines with increasing drainage area as a result of dilution from waters draining urban and other land not actually under construction. Observations demonstrate that sediment storage occurs on construction sites as well as in valley bottomlands. Actual yields from a given unit surface may be even larger than those derived from measurements in streams. Data on erosion from roadcuts in Georgia, when converted to soil loss per area, result in sediment yields similar to those from building sites: 50,000–150,000 t/mi2/yr (20,000–50,000 t/km2/yr), and local measurements indicate depths of erosion on roadcuts of 0.1–0.2 ft (3–6 cm) over time intervals of generally less than one year. Imposition of large quantities of sediment on streams previously carrying relatively small quantities of primarily suspended material produced deposition of channel bars, erosion of channel banks as a result of deposition within the channel, obstruction of flow and increased flooding, shifting configuration of the channel bottom, blanketing of bottom‐dwelling flora and fauna, alteration of the flora and fauna due to changes in light transmission and abrasive effects of, sediment, and alteration of species of fish due to changes produced in the flora and fauna upon which fish depend. Analysis of building permit records showed that: (1) 50% of construction sites were open for eight months, 60% for nine, and 25% for more than one year; (2) contrary to expectations, construction activity is practically constant throughout the year, with about 84% of all sites being open in any one month; (3) the average size of a construction site for one permit‐bearing building is 14,400 ft2 (1340 m2), a value that remained constant during the last decade. These findings, combined with statistics on highway construction, indicate that in four Maryland counties covering expanding metropolitan regions adjacent to Baltimore a...

Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein-Hubbell-Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles. 5O3

Abstract:Runoff generation in a Mediterranean carbonate rock environment-the Judaean Mountains, West Bank and Israel-was studied by a 2 day sprinkling experiment on an 18 ð 10 m 2 plot. During the first day, three spells of 24Ð1 mm h 1 (12Ð3 mm), 27Ð8 mm h 1 (12Ð3 mm) and 14Ð4 mm h 1 (12Ð5 mm) were applied, almost saturating the runoff plot. On the second day, 20 h later, two spells of 18Ð7 mm h 1 (34Ð3 mm) and 35Ð2 mm h 1 (25Ð4 mm) followed. Surface runoff was a combination of infiltration excess runoff from rocky portions of the plot and saturation excess runoff from areas covered by soil. Soil saturation was accelerated by lateral runoff from adjacent rocky areas. Once the plot was saturated, 80-90% of the applied rainfall became surface runoff. Significantly different concentrations of chloride, sulphate, and nitrate ions in the sprinkled water on two successive days served as tracers. During the second day a two-component hydrograph separation pointed to the importance of shallow reservoirs playing active parts in runoff generation. About 14% of the flow collected during the second day originated from water applied during the first day. During the second day of sprinkling, both water sources obviously mixed in saturated soil reservoirs and contributed in variable percentages to surface flow. It is hypothesized that, at least during high-magnitude rainfall, the steep rocky slopes investigated might be regarded as flood-generating zones rather than areas of pronounced recharge into the underlying Yarkon-Taninim aquifer.

The Nahal Yael basin is underlain chiefly by schist, amphibolite, and granite. Thin (generally <1 m thick), grussy colluvium which covered the lower portions of granitic hillslopes in the late Pleistocene has now been stripped completely, causing marked contrasts in outcrop morphologies, even where there is no contrast of fracture density or petrologic characteristics. Formerly mantled slopes are now smooth and crumbly, and lack desert varnish. Previously unmantled slopes are rough and craggy, and varnished but little weathered. Such stripping suggests a change from a semiarid to a drier and/or warmer climate. Slopes underlain by amphibolite responded similarly to the climatic change, but the amphibolite was more deeply weathered, and the colluvium was only partially stripped. The least stripping of colluvium occurred on schist hillslopes, partly because schist outcrops require more rain to generate runoff, and partly because angular blocks of schist require larger flows for transport, compared to other slope lithologies. The stream subsystem responded to the climatically induced changes in the discharge of water and sediment from the hillslopes. Increase in sediment yield caused valley alluviation in the early Holocene, and a decrease in sediment yield later in the Holocene caused entrenchment of the valley fill. More granite and amphibolite gravel-size particles are transported now than when the hillslopes were extensively mantled. Dense networks of trails are not common on Holocene geomorphic surfaces, but are present on remnants of Pleistocene surfaces.

The palaeohydrology of Nahal Zin, a 1400 km 2 catchment in the hyperarid Negev Desert, is inferred from slackwater deposits and palaeostage indicators in a canyon near its lower end. The palaeoflood record, augmented by the instrumental and historical records of the last decade, includes 28 floods ranging from 200 to 1500 m 3 s À1 over the last 2000 years. This helps to reanalyse the frequency of floods in this drainage system. The clusters of floods around 1000 years BP and again during the last 60 years are characterized by high flow magnitudes.Periods with many floods correspond well to periods with high Dead Sea levels and are probably relatively wet periods, while periods with few floods correspond well to low Dead Sea levels indicating a drier climate. Fluctuations in the frequency of floods are typical of periods of transition from one climate regime to another.