Patrick T. Pringle

Monument Creek debris flow, 1984: Implications for formation of rapids on the Colorado River in Grand Canyon National Park

A recent debris flow in Monument Creek illustrates the nature of debris flows in small tributaries and their hydrologic effects on the Colorado River in Grand Canyon National Park. A debris avalanche originated in the Permian Esplanade Sandstone of the Supai Group during intense rainfall on July 27, 1984, and fell 600 m into Monument Creek, forming a 7-m-high barrier across the channel. The subsequent debris flow traveled 4.5 km to the Colorado River and achieved velocities of 3.4 to 4.0 m/s and a peak discharge of 100 to 120 m 3 /s. The flow consisted of a main pulse followed by subsequent pulses of debris flow or hyperconcentrated flow. The main pulse moved boulders as large as 2.7 m in diameter, and deposition at the mouth of Monument Creek enlarged the fan surface and significantly constricted the Colorado River. Most of the major rapids on the Colorado River in Grand Canyon National Park appear to be maintained by episodic debris flows.

A detailed chronology of the most recent major eruptive period at Mount Hood, Oregon

The most recent eruptive period of Mount Hood volcano, the Old Maid eruptive period, was characterized by volcano-hydrologic events (hydrologic events initiated by volcanic activity) which resulted in extensive lahar inundation in the White, Sandy, and Zigzag River drainages and produced a lithic pyroclastic flow which traveled at least 9 km down the White River from the vent area at Crater Rock. Interpretations of downstream textural changes in deposits indicate that one lahar reached as far as Tygh Valley (65 km from the vent) before transforming into a lahar runout (hyperconcentrated flow). The runout inundated Tygh Valley and flowed into the Deschutes River, 75 km (flow path) from the volcano. A single lahar traveled more than 30 km down the Sandy River before transforming to a runout. Correlative sands and gravels are found as far as the apex of the Sandy River delta, more than 80 km from the volcano; these suggest that the flow underwent minimal attenuation of stage height throughout the length of Sandy River. Approximate dates ranging from 1760 A.D. to 1810 A.D. for various Old Maid-age events are inferred from dendrochronologic studies of old growth trees. There have been no apparent major topographic changes in the vent area since the end of Old Maid-age activity, enabling the events of the Old Maid eruptive period to be used as a model for future eruptive activity.

Progress made in understanding Mount Rainier's hazards

At 4392 m high, glacier-clad Mount Rainier dominates the skyline of the southern Puget Sound region and is the centerpiece of Mount Rainier National Park. About 2.5 million people of the greater Seattle-Tacoma metropolitan area can see Mount Rainier on clear days, and 150,000 live in areas swept by lahars and floods that emanated from the volcano during the last 6,000 years (Figure 1). These lahars include the voluminous Osceola Mudflow that floors the lowlands south of Seattle and east of Tacoma, and which was generated by massive volcano flank-collapse. Mount Rainiers last eruption was a light dusting of ash in 1894; minor pumice last erupted between 1820 and 1854; and the most recent large eruptions we know of were about 1100 and 2300 years ago, according to reports from the U.S. Geological Survey.