The purpose of this research project is to determine the origin of the materials used to construct the Black Hills Dam in order to restore the landscape to pre-dam conditions. The Black Hills Dam site is located in northern Scottsdale, Maricopa County, at 33.75° North, 111.80° West. The goals of this project are to characterize the surficial deposits and local geology of the dam site. This report presents our findings, interpretations and conclusions based on background research, a site visit to the dam site, and technical discussions with the City of Scottsdale engineer and planners.

Throughout the southwestern United States, vegetation in what historically was grassland has changed to a mixture of trees and shrubs; exotic grass species and undesirable shrubs have also invaded the grasslands at the expense of native grasses. The availability and amount of soil nutrients influence the relative success of plants, but few studies have examined fire effects on soil characteristics in a temporal, spatial, and species group-specific fashion. Likewise, few studies have tied fire effects and ecological aspects to the underlying geology. Our research investigates the effects of fire events on selected soil characteristics pH, nitrate (NO3-), plant-available phosphorus (PO4-3), and total organic carbon (TOC) on native grass-, exotic grass-, and mixed grass-dominated plots distributed on four different geological surfaces. Treated and control plots were sampled prior to burn treatment and at intervals after the burns. In addition to new geologic mapping of the study areas, results indicate the geologic substrate is the most important variable for explaining pH, NO3- and PO4-3 values in the soils. Dominant grass type – native, non-native, or mixed – had little effect on the response of soil geochemistry to fire events: post-burn results indicate vegetation was a significant factor only for TOC. Recovery to pre-burn levels varies with characteristic: there were no significant initial differences between vegetation types, but significant differences in NO3-, PO4-3, and TOC amounts occur as a result of fire events, geological characteristics, and time. The research helps identify the soil response to fire and the recovery times of soil characteristics, further defines which fire frequency is optimal as a management strategy to maximize soil macronutrient contents, and illustrates the important role geology plays in grassland ecosystems.

A series of small earth movements occurred along the slopes of State Route 87 at about mile marker 224 (between the Bush Highway and Route 188) throughout the winter of 2007‐2008, culminating with a landslide on Friday, 21 March 2008. This landslide buckled the southbound lanes, displaced the northbound lanes, and closed the highway for nearly a week. The mass movements occurred on slopes that were constructed with re‐vegetated, laid back slopes; soil nail walls; and rip rap‐lined channels. However, our reconnaissance mapping indicates that most, if not all, of the slope movements are located within a much larger, older landslide adjacent to, and cut by, SR‐87. No specific trigger for the landslides was immediately apparent, although a combination of factors (e.g., precipitation, groundwater levels, etc.) may have contributed to conditions for the slope failure. Headscarps of various types of landslides are present along both sides of the highway. Results from our preliminary investigation indicate that up to three other paleo‐landslides may be present in the immediate vicinity. The full extent and nature of these landslides are unknown and require further investigation to evaluate their potential to be reactivated and risk they pose to the highway.