Theodore S. Melis — Page 3

USGS1997

Initiation of debris flows in tributaries of the Colorado River in Grand Canyon, Arizona

Debris flows are initiated in tributaries of the Colorado River in Grand Canyon when intense rainfall causes failures in colluvium and (or) bedrock. Most debris flows occur in the summer during localized convective thunderstorms with rainfall intensities as high as 40 mm/hr. Rarer and larger debris flows occur during unusually warm frontal storms in winter. Hourly precipitation data suggest that storms that cause debris flows terminate with a period of intense rainfall, a characteristic that complicates the use of daily rainfall records in assessing debris-flow hazard. Recurrence intervals for 1-day rainfall associated with 37 recent debris flows range from <1 to >50 years, with most <10 years. Recurrence intervals for the multi-day rainfall of storms associated with debris flows range from <1 to 158 years, but most were >10 years. The low recurrence intervals of debris-flow producing rainfalls, compared with the 10-50 yr recurrence intervals for most debris flows, underscores the co-dependence of debris-flow initiation on geologic factors, including bedrock type and antecedent soil-moisture conditions. The primary geologic factor influencing debris-flow initiation in Grand Canyon is the exposure of shale units at heights >100 m above the river. Exposed shale bedrock fails readily, either producing debris flows directly or contributing source material to wedges of colluvium that may fail later. Shales also provide silt- and clay-size particles that in part determine the rheological properties of debris flows.

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USGS1996

When the blue-green waters turn red: Historical flooding in Havasu Creek, Arizona

Havasu Creek, the second largest tributary of the Colorado River in Grand Canyon National Park, attracts numerous visitors each year owing to its spectacular scenery. Perennial streamflow seldom exceeds 2 cubic meters per second (m 3 /s), but supports important stands of riparian vegetation, forms unique travertine pools, and spills over spectacular waterfalls. Havasu Canyon is home to the Havasupai Tribe, consisting of 423 members living in Supai, Arizona. Flooding in Havasu Creek poses a hazard to both visitors and residents of Supai. Frequent, large floods occurred in winter and summer during the late 19th and early 20th centuries; the largest occurred in January 1910. Smaller, summer floods occurred between 1935 and 1990. In September 1990, the largest flood in Havasu Creek since 1935, and possibly 1910, was generated by intense thunderstorms that lasted several days. The 1990 flood peaked at 575 m3/s, caused severe damage to Supai, killed hundreds of ash trees (Fraxinus sp.), and altered travertine deposits in lower Havasu Canyon. Smaller floods in July 1992 and February 1993 also damaged Supai, eroded waterfalls, destroyed riparian vegetation, filled pools with gravel, and deposited coarse debris in the Colorado River. Most ash trees in Havasu Canyon germinated after 1940; peak recruitment occurred in the late 1960s and early 1970s, possibly in response to human disturbance. Nearly 80 percent of historical Havasu Creek floods have occurred during or immediately following El Niño years. Recent 1990s flooding reflects the flood regime of the first third of the 20th century, and frequency of intense daily precipitation at stations near Havasu Creek has followed patterns in recent flood frequency.

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