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The temporal and spatial changes in chemical and biological properties of Lake Mead have been investigated, thereby indicating the sources of water pollution and the time of highest pollution potential. Planktonic organisms have been shown to indicate the presence of water problems. Macro- and micro-nutrient analyses have shown that primary productivity is not inhibited by limiting concentrations. A mathematical model has been developed, tested with one set of independent data, and shown worthy of management utility. Although the model works very well for the Lake Mead area, the physical reality of the Multiple Linear Regression equation should be tested on independent data.

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No description available.

Traditional flood-frequency techniques are based on the assumption that the observed flood record represents a sample that has been drawn from a single climatically homogeneous population of floods. A hydroclimatic approach was used to evaluate this assumption by identifying the circulation patterns and atmospheric flood-generating mechanisms which control the temporal and spatial variability of flooding. Mean monthly discharges and instantaneous peak flows of the partial duration series were analyzed for thirty gaging stations in the climatically sensitive, semiarid, Gila River basin for the period 1950 to 1980. Correlation fields and composite maps were constructed to define the relationship between 700 mb height circulation anomalies and mean monthly streamflow. Individual flood events were linked to climate by analyzing daily synoptic weather maps and classifying each flood event into one of eight hydroclimatic categories on the basis of the atmospheric mechanisms which generated each flow. The analysis demonstrated that floods and anomalously high streamflow in the Gila River basin originate from a variety of atmospheric processes which vary spatially, seasonally, and from year-to-year. The mechanisms most important for generating floods included winter fronts, cutoff lows, tropical storms, snowmelt, and widespread and localized summer monsoon-related circulation patterns. When flood discharges were grouped into hydroclimatically homogeneous categories, histogram plots of their frequency distributions exhibited means and variances that differed from those of the overall frequency distribution of the entire flood series. The means of the discharges generated by frontal precipitation and tropical storms tended to plot above the mean of the overall series, while the means of floods generated by snowmelt tended to plot below the overall mean. Flood estimates computed from a series containing mixed distributions were not the same as flood estimates computed from climatically homogeneous subsets of the same series. These results have implications for traditional flood-frequency analysis and other stochastic methods of analyzing hydrologic time series. The hydroclimatically-defined subgroups in the flood series of the Gila River basin indicate that nonhomogeneity and nonstationarity can be imparted to a hydrologic time series by differing atmospheric mechanisms alone.

A study was conducted to test for the presence of heavy metals (Cu, Mn, and Zn) in surface sediments of the Puerco River channel in the aftermath of a toxic spill in 1979 near Church Rock, New Mexico. Analysis of samples from five sites downstream from the spill showed that these substances were not present in unusually large amounts, though an increasing gradient of metal concentration with distance downstream was revealed. Statistical analysis revealed the Cu, Mn, and Zn were associated with clay and silt, soil organic matter, organic carbon, and carbonates, all of which existed as extraneous, uncontrolled variables. Adjusted metal concentrations, obtained with covariate analyses, confirmed the increasing gradient downstream. Clay and silt also increased downstream. Some toxic substances may have leached into the riverbed; possible mechanisms for this process are also discussed and further study to substantiate or disprove this hypothesis is recommended.

The Santa Cruz River in southeastern Arizona is typical of large rivers of the semiarid southwestern United States. The 390-kilometer long river displays great variability in morphology, hydraulics, hydrologic, conditions, and bank stability, reflecting incomplete integration of the drainage system and the disequilibrium that is often characteristic of desert channels. Spatial variability is paralleled by temporal variability at all scales, from instantaneous conditions to millenia of geologic history. The alluvial history of the Santa Cruz River drainage basin suggests that transverse sediment transport toward the valley across alluvial fans has dominated fluvial processes for most of the history of the basin. No evidence is apparent for external drainage of the basin before the beginning of Fort Lowell Formation deposition, about 2.0 to 2.5 million years ago. The mid to late Pleistocene history of the system is poorly understood because of the paucity of the geologic record for this period. Several episodes of terrace formation apparently represent climatically induced sediment pulses and subsequent incision. The Holocene record of the Santa Cruz River suggests a major change in hydrologic regimen about 5,000 years ago when deposits indicate a substantial decrease in stream power. The last 2,500 years are marked by a sequence of channel cut and fill episodes similar to the historic episode of channel entrenchment that began in the 19th century. The hydrologic regimen of the Santa Cruz River is dominated by the occurrence of floods in direct response to precipitation. Three major storm types—monsoonal, frontal, and dissipating tropical—account for most streamflow. The storms cause floods with distinctly different characteristics. Monsoonal storm-caused floods may have quite high peak discharges, but are local and of short duration. Frontal and tropical storms cause floods of longer duration and flood volume and are more efficient agents of sediment transport and geomorphic change than are those caused by monsoonal storms. Geologic controls are the major factor governing the spatial variability of channel morphology and change. Hydrologic factors control the temporal variability of channel processes. Frontal and tropical storms are more likely to produce floods capable of causing and maintaining a continuously entrenched main channel. Floods caused by monsoonal storms do not appear capable of maintaining such a system.

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No description available.

No description available.