Las Vegas images
A comment by Laura R. Musacchio
The images of the growth of Las Vegas are great. It is so difficult for people to visualize the impacts of development over several generations. These images fill an important gap -- visually persuasive images that can change how people see the world. I encourage the authors of this paper to create more image sequences for major metropolitan areas of the West. Phoenix would be a good candidate because it is so decentralized.
Urban geologic mapping
A comment by Scott Lundstrom
As part of the Las Vegas Area Urban Corridor Project, I am mapping the Quaternary geology of the Las Vegas 1:100,000 sheet, which essentially provides a record of geomorphic and hydrologic response to past climate change, and documents surficial soil and deposit distribution prior to urbanization. It is good to see the digitized record of urban growth reported by Acevedo et al, and Sayler - analyses that combine such data with our digitized geologic mapping will be able to address many issues related to the urban impact on the landscape. One example is flood hazard. Where areas of alluvial fans, eolian deposits, and past groundwater discharge deposits that the dominate the surficial cover of the Las Vegas Valley are covered by housing developments, the infiltration characteristics of the surface are changed. Infiltration is generally reduced and runoff increases because pavement and rooftops generally have no infiltration capacity. In contrast, the fine-sand- dominated eolian veneer on Quaternary deposits has a greater infiltration capacity, but one which varies with its depth and somewhat inversely with the amount of soil caliche development related to the age and geologic history of the underlying surficial materials, as documented by geologic mapping. Thus the spatially varying change in infiltration and runoff potential for specified precipitation scenarios can be evaluated to address the change in flood hazards with urbanization. This real effect was illustrated in November, 1996, when a relatively minor regional rainfall event of about 2 cm over 3 hours produced disruptive flooding in central urban areas of Las Vegas. In contrast, no evidence of runoff was observed in any of the natural channels of not-yet- urbanized alluvial fans draining higher areas that likely received greater orographically enhanced rainfall. On the other hand, documentation of enhanced runoff through analysis of both urban mapping and surficial geology might potentially increase water supply to the urban area by documenting increased recharge back into the Colorado River. Scott Lundstrom National Cooperative Geologic Mapping U.S. Geological Survey Denver, CO
Impacts of urbanization
A comment by Laura R. Musacchio
Questions raised by Scott's good comment: I am a landscape planner/landscape ecologist, so my thoughts are reflective of my background. It is very plausible the increased amount of impervious pavement in Las Vegas could enhance runoff (and recharge) into the Colorado River and provide more water for Las Vegas. However, the decreased infiltration of water into the city's soils may also prevent adequate recharge of an aquifer which is below the city (if there is one to begin with, my knowledge is limited.). Or perhaps with all the lawn watering, there is plenty of ground water recharge? The most difficult aspect of these highly-urbanized ecosystems in an the desert is predicting how they will behave given different natural or human pertubations. We have only begun to scratch the surface of understanding the dynamics of these systems. I do not think these desert cities have been around long enough for us to have seen all the dynamics of these systems (however, the fate of the Anazazi pueblos may be a good place to start). My guess is that they will behave as non-linear systems.