NOAA's National Weather Service Weather Forecast Office

SYLVAN PASS MUDSLIDE/DEBRIS FLOW OF JULY 18, 2004

Photo 1.  The main mudslide/debris flow totally covering the road and engulfing a vehicle.                                                                                                              NOAA Photo by Paul Skrbac

            Monsoonal moisture coupled with afternoon instability in western Wyoming resulted in

numerous thunderstorms developing during the afternoon and evening of July 18th.  Several rounds

of storms with locally heavy rainfall moved through eastern portions of Yellowstone National Park

from 3 p.m. MDT to 10 p.m. MDT.  Rain gages in the area reported rainfall rates from a half an 

inch to almost two inches of rain an hour between 5 p.m. and 7 p.m. MDT. As a result, the heavy 

rain quickly saturated the soils of over steepened slopes and the result was a mass wasting event

usually called a mudslide or debris flow.  The worst of a series of mudslides/debris flow occurred 

around 8 p.m. MDT and was located around 6 miles west of Yellowstone National Park’s east

entrance. Up to 4 feet of mud, rock, and debris totally covered the road and three vehicles were

trapped in the area. 16 people were rescued and there were no known injuries.  

Photo 2.  Three cars covered by rocks, mud, and other debris.                     NOAA Photo by Paul Skrbac 

To fully understand and appreciate what happened on July 18^th, one must look at the
geomorphology or landscape processes that aided in the resulting mass wasting event.   The
major player in this event was a hillside that was over steepened by natural and possible manmade
processes.  The steep slope of the hillside contained highly erodable volcanic rock and
accompanying highly permeable soil which contributed the instability of the hill side.  Human
activities like road maintenance at the base of the hillside may, over time, further over steepened
the slope and made the hill side even more unstable.  However precarious, the slope achieves an
angle, called the angle of repose, in which a balance between opposing natural and manmade
forces occurs.

A mass movement event takes place when the balance of forces within a hillside is
disrupted and the slope becomes steeper than the angle of repose.  The factors which most
influence the stability are gravity, water, sensitive soils, and a triggering event.  Obviously, the
triggering event during the mass wasting event of July 18th was a tremendous amount of rainfall
in a short period of time (2 hours or less).  The rainfall easily infiltrated the sensitive soils of the
hill side.  The soil material then became less cohesive as a whole and flowed down the unstable
slope as a mudslide and debris flow.

Picture 3.  Mass wasting over an unstable slope with highly saturated and non-cohesive soils.
Smaller mudslide near the major mudslide/debris flow near Sylvan Pass.
                                                                                                                NOAA Photo by Paul Skrbac