Dru Germanoski

Influence of late Holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central Nevada

Stratigraphic, geomorphic, and paleoecological data were collected from upland watersheds in the Great Basin of central Nevada to assess the relationships between late Holocene climate change, hillslope processes and landforms, and modern channel dynamics. These data indicate that a shift to drier, warmer climatic conditions from approximately 2500 to 1300 YPB led to a complex set of geomorphic responses. The initial response was massive hillslope erosion and the simultaneous aggradation of both side-valley alluvial fans and the axial valley system. The final response was fan stabilization and axial channel incision as fine-grained sediments were winnowed from the hillslope sediment reservoirs, and sediment yield and runoff processes were altered. The primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment, suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision. The magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side-valley alluvial fans. Radial fan profiles suggest that during fan building, fan deposits temporarily blocked the flow of sediment down the main stem of the valley, commonly creating a stepped longitudinal valley profile. Stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition. In contrast, reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition. Thus, modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago.

ASYNCHRONOUS TERRACE DEVELOPMENT IN DEGRADING BRAIDED CHANNELS

Terrace remnants are commonly used to reconstruct longitudinal profiles of rivers and floodplains, and to establish temporal correlations of events in fluvial systems. In most cases, it is assumed that the terrace remnants represent time-equivalent surfaces. Our observations of terrace formation in flume experiments and in a degrading braided river, Ash Creek, Arizona, suggest that this assumption is not always valid. Degradation resulted from a reduction in upstream sediment delivery to braided channels. In both the flume and Ash Creek, degradation in the upstream reach produced a number of inset terraces, while the production of sediment in the degrading reach simultaneously caused further aggradation downstream. Thus, stratigraphically lower surfaces in the upstream reaches are temporally equivalent to higher surfaces in downstream reaches. The downstream progression of the wave of incision produced more terraces upstream than downstream, and terrace surfaces could not be correlated on the basis of rel...

GEOMORPHIC RESPONSE TO WILDFIRE IN AN ARID WATERSHED, CROW CANYON, NEVADA

On August 28, 1981, the Crow Canyon drainage basin in central Nevada was burned by a lightning-generated wildfire that destroyed the vegetation cover consisting primarily of juniper trees, sagebrush, and desert grasses. The geomorphic impact of the wildfire was assessed on the basis of aerial photography, measurements of sediment movement on hillslopes using charred tree trunks as erosion indicators, and surveys of the valley floor, axial channel, and alluvial fan. Aerial photographs indicate the valley floor was untrenched prior to the fire. The combination of foliage destruction and heavy runoff in the spring following the wildfire initiated channel downcutting that has now reached as much as 3.9 m in depth. Entrenchment of the valley-fill in the lower 2.2 km of the drainage network produced as much as 48, 142 m3 of sediment. Much of the channel incision occurred during 1982 and 1983, years characterized by above-normal precipitation. Approximately 17,608 m3 of sediment were deposited on a preexisting a...

Spatial and temporal variations in the transport and storage of trace metal contaminants in the upper Rio Pilcomayo, southern Bolivia

Mining of the Cerro Rico de Potosi precious metal-polymetallic tin deposits of Bolivia began in 1545 and has led to severe contamination of water and sediments of the upper Rio Pilcomayo drainage system. Detailed geomorphological, stratigraphic and geochemical analyses show that the downstream transport and deposition of sediment-borne trace metals from Cerro Rico vary among three types of river reaches. Reach types, called process zones, are characterised by a semi-homogeneous set of landforms and geomorphic processes. The potential for trace metals to be remobilised and negatively impact the riverine ecosystem also differs between these process zones. The analysis, then, demonstrates that the assessment of trace metal contamination within rivers that drain the eastern flank of the Andes, and other high relief terrains, requires an interdisciplinary approach that integrates an understanding of both geomorphological and geochemical factors.