Geology topics

Research about Western Cascades

Source-linked reports with geographic coverage including Western Cascades.

3 recordsLinked to original sources

USGS2020

Runoff-initiated post-fire debris flow Western Cascades, Oregon

Wildfires dramatically alter the hydraulics and root reinforcement of soil on forested hillslopes, which can promote the generation of debris flows. In the Pacific Northwest, post-fire shallow landsliding has been well documented and studied, but the potential role of runoff-initiated debris flows is not well understood and only one previous to 2018 had been documented in the region. On 20 June 2018, approximately 1 year after the Milli fire burned 24,000 acres, a runoff-initiated debris flow occurred on the flanks of Black Crater in the Oregon Cascade Range. The debris flow was initiated via dispersed rilling on > 30-degree slopes near the crater rim and traveled > 1.5 km downslope. We measured exceptionally low soil infiltration rates at the study site, likely due to high burn severity during the Milli fire. Based on nearby 5-min rain gage data, we quantified rainfall rates for the storm event that triggered the debris flow. Our results show that peak 15-min rainfall rates were 25.4 mmh −1 , equaling or exceeding the measured infiltration rates at the study site, which had a geometric mean of ~ 24 mmh −1 . Field mapping shows that high burn severity resulted in the initiation of the debris flow and that convergent and steep topography promoted the development of a debris flow at this site. As wildfires increase in frequency and intensity across the western USA, the Pacific Northwest could become more susceptible to runoff-initiated debris flows. Therefore, characterization of the conditions that resulted in this debris flow is crucial for understanding how runoff-initiated debris flows may shape terrain and impact hazards in the Pacific Northwest.

Oregon↗

USGS2002

Ecological effects on streams from forest fertilization: Literature review and conceptual framework for future study in the western Cascades

Fertilization of forests with urea-nitrogen has been studied numerous times for its effects on water quality. Stream nitrogen concentrations following fertilization are typically elevated during winter, including peaks in the tens-of-thousands of parts per billion range, with summer concentrations often returning to background or near-background levels. Despite these increases, water-quality criteria for nitrogen have rarely been exceeded. However, such criteria are targeted at fish toxicity or human health and are not relevant to concentrations that could cause ecological disturbances. Studies of the responses of stream biota to fertilization have been rare and have targeted either immediate, toxicity-based responses or used methods insensitive to ongoing ecological processes. This report reviews water-quality studies following forest fertilizations, emphasizing Cascade streams in the Pacific Northwest and documented biological responses in those streams. A conceptual model predicting potential ecological response to fertilization, which includes effects on algal growth and primary production, is presented. In this model, applied fertilizer nitrogen reaching streams is mostly exported during winter. However, some nitrogen retained in soils or stream and riparian areas may become available to aquatic biota during spring and summer. Biological responses may be minimal in small streams nearest to application because of light limitation, but may be elevated downstream where light is sufficient to allow algal growth. Ultimately, algal response could be greatest in downstream reaches, although ambient nutrient concentrations remain low due to uptake and benthic nutrient recycling. Ground-water flow paths and hyporheic processing could be critical in determining the fate of applied nitrogen. A framework is provided for testing this response in the Little River watershed, a tributary to the North Umpqua River, Oregon, at basic and intensive levels of investigation.

Oregon↗

USGS1993

Heat flow from four new research drill holes in the Western Cascades, Oregon, U.S.A.

Conceptual models of the thermal structure of the Oregon Cascade Range propose either (1) a narrow zone of magmatic heat sources, flanked by shallow heat-flow anomalies caused by lateral ground-water flow; or (2) a wide zone of magmatic heat sources, with localized, generally negligible ground-water effects. The proposed narrow heat source coincides with the Quaternary volcanic arc, whereas the wider heat source would extend 10–30 km west of the arc. To test the models, four new heat-flow holes were sited west of the Quaternary arc but within the area of the proposed wide heat source. The sites are separated from the Quaternary arc by topographic divides and (or) major river valleys, so that heating by regional-scale ground-water flow seems unlikely. Measured heat flow (76±5 mW mm −2 ) was significantly lower than the values predicted by interpolation from an existing heat-flow contour map (95±7 mW m −2 ). The lower values are consistent with a narrow zone of magmatic heat sources. The complete heat-flow data set consists mostly of shallow (100–200 m) data and defines lobate highs around hot-spring groups in the Western Cascades. However, all of the deepest holes in the study area (44°–45°15′N) show hydrologic gradient disturbances extending to depths >200 m, so the shallow data must be extrapolated with caution.

Oregon↗