Variability of Sediment Removal in a Semi-Arid Watershed

Field and documentary data from Walnut Gulch Watershed, an instrumented semiarid drainage basin of approximately 150 km2 (57 mi2) in southeastern Arizona, show that 83% of the alluvium removed from the basin during a 15‐year erosion episode beginning about 1930 was excavated from the highest‐order stream. The amount of alluvium removed in the erosion episode would have been equal to a covering of about 4 cm (1.6 in) over the entire basin. The rate of sediment removal during the erosion episode was 18 times greater than the rate of present channel sediment transport. Production of sediment from slopes and channel throughput at present rates are approximately equal, and refilling will not occur under present conditions. The channel forms left by the massive evacuation of sediment impose controls on the spatial distribution of tractive force and total stream power that make renewed storage of sediment likely in only a few restricted locations. Modern instrumented records of a decade or more provide an inadequate perspective on long‐term sediment movement. Field and documentary data from Walnut Gulch Watershed, an instrumented semiarid drainage basin of approximately 150 km2 (57 mi2) in southeastern Arizona, show that 83% of the alluvium removed from the basin during a 15‐year erosion episode beginning about 1930 was excavated from the highest‐order stream. The amount of alluvium removed in the erosion episode would have been equal to a covering of about 4 cm (1.6 in) over the entire basin. The rate of sediment removal during the erosion episode was 18 times greater than the rate of present channel sediment transport. Production of sediment from slopes and channel throughput at present rates are approximately equal, and refilling will not occur under present conditions. The channel forms left by the massive evacuation of sediment impose controls on the spatial distribution of tractive force and total stream power that make renewed storage of sediment likely in only a few restricted locations. Modern instrumented records of a decade or more provide an inadequate perspective on long‐term sediment movement

Channel Instability in a Braided Sand Bed River

The Gila River of central Arizona is representative of braided, sand bed rivers in alluvial valleys that have inherent unstable behavior and destructive channel migration. The 112-year record of channel conditions along a portion of the Gila River provides data for the construction of locational probability maps for main flow channels. Zones of stability and hazardous instability alternate with each other at 3.2 km (2 mi) intervals. During the past century the overall sinuosity of the main flow channel has remained close to 1.18, despite numerous changes in actual location. Spatial and temporal variation of sinuosity have occurred in subreaches as a result of sedimentation behind a dam and fluctuations in the density of phreatophyte growth, which both affect the hydraulics of flood flows. Unstable zones of the channel correspond to the surface of the sediment wedge behind the dam and areas dense phreatophyte growth. Stable zones correspond to areas controlled by bedrock or man-made structures, as well as locations determined by these external factors plus the requirement to maintain a consistent sinuosity. Channels such as that of the Gila River do not meet most assumptions of equilibrium and are best understood through probabilistic approaches with an assumption of catastrophic adjustment

A Probabilistic Approach to the Spatial Assessment of River Channel Instability

The deterministic approach to the analysis of river channel instability has not proved to be a completely useful basis for geographic predictions of channel behavior. Economic estimates for benefits of structural channel control projects commonly account for flood inundation, but in arid and semiarid regions these estimates are incomplete because they fail to take into account destructive channel migration and erosion. As a solution, a method whereby historical records of channel locations are reduced to spatially defined probabilistic functions allows calculation of the probability that given parcels of near-channel terrain will be destroyed by erosion. The probability of erosion for any given parcel over a given period of time is directly proportional to the sizes of the annual floods during the period and inversely proportional to two distance measures: distance upstream and distance laterally to the channel. In a test of the probabilistic geographic method using data on the locations of Rillito Creek, Arizona, from 1871 to 1978, erosion probability maps accurately characterized the locations of observed changes. In a 50-year simulated period, erosion ultimately produced economic losses that were 5 times greater than potential inundation losses

The Impact of Suburbanization on Fluvial Geomorphology

Analysis of aerial photography for the period 1950–1971 and field data collected from 1970 to 1974 indicate that in the Denver area suburban development has caused significant changes in fluvial systems. By first introducing large quantities of sediment and later by increasing surface runoff, suburban development leads to an expansion of floodplains followed by downcutting of streams. As areas of suburban development increase, greater percentages of stream lengths are dominated by transportation, and lesser percentages are dominated by erosion and deposition

Network Characteristics in Suburbanizing Streams

Analysis of hydrologic, geomorphic, and suburbanization data from a small instrumented drainage basin near Iowa City, Iowa, indicates that channel networks are radically altered when suburban development overtakes a drainage basin. Changes in channel networks are such that the network becomes much more efficient in collecting water quickly, so that lag time and kurtosis of storm hydrographs are altered to produce the familiar flash floods of urban areas. The data show that network changes are closely associated with lag time and kurtosis of storm hydrographs and suggest that corrective measures should be concentrated on the internal links of the network. Changes in characteristics of channel networks should be considered in addition to changes in areas of impervious surfaces when the hydrologic impact of suburbanization is assessed

Cirques as Glacier Locations

A comparison between the 319 cirques that contain glaciers and a sample of 240 empty cirques in the Rocky Mountains shows that in the present climatic situation, landforms are strong factors in determining the locations of glaciers. An optimum glacier location is a large cirque facing northeast, with a planimetric shape of width greater than length, high steep walls, a pass located to the windward, and a peak to the southwest. Glaciers survive in the present climatic conditions because of a geomorphic feedback system, whereby glaciers are protected by cirque forms that owe their morphology to glacial processes