WSSPC FIELD TRIP
Monday September 12, 2005
8:00 am to 2:00 pm
Leaves from and returns to The Grove Hotel

NATURAL HAZARDS OF THE BOISE AREA

Leaders:

Bill Phillips, Idaho Geological Survey
Stephen Weiser, Idaho Bureau of Homeland Security

The Boise area, home to about 23% of Idaho’s population, is subject to a variety of natural hazards including earthquakes, flooding, landslides, and wildfires. This field trip examines the interaction of these natural hazards with the dynamics of rapid population growth and development. The trip is designed to complement themes of the conference technical sessions.  The theme of  “Quantifying the Hazard” is addressed by presenting evidence for active faulting and the historical record of flooding, mass-wasting, and wildfires. The theme of “Changing Behaviors” is introduced by noting examples of recent development in sensitive areas such as floodplains and hillslopes. Examples illustrating differences between municipal and county land use planning and building code enforcement are highlighted. Under the theme “Setting Priorities,” the debate between focusing hazard mitigation efforts on flood damage vs earthquake damage is introduced.

Boise is situated between two fault zones with evidence of Quaternary (past 1.6 million years) activity. The Western Snake River Plain fault system lies southwest of the city. This zone consists of numerous northwest-trending normal faults with little surface expression that offset older Quaternary deposits. To the north of the city is the Western Idaho fault system, a group of north-trending normal faults. The latest movement on this system was along the Squaw Creek Fault, near Emmett, Idaho, about 6700 years ago. The field trip visits the Squaw Creek area and the nearby Black Canyon Dam. Much of Boise was built without consideration of seismic hazards. The field trip views examples of housing stock in the Boise area and discusses the vulnerability of these structures to earthquakes.

Flooding is an important hazard along the Boise River and its tributaries. Despite upstream regulation by three large dams, the river reaches a flood stage of 7000 cfs at Boise every year, and in June of 1983 had a flow of 9840 cfs. Property damage and inconvenience caused by flooding occurs most years. Encroachment and development in the river floodway is aggravating this situation. Future floods above 10,000 cfs will result in substantial expense to the community. Flash flooding and debris flows often occur as the result of summer thunderstorms in small creeks draining the Boise foothills. Development along the base of the Boise foothills is increasing this hazard. Thunderstorm-generated wildfires in these steep drainages greatly increase the potential for damaging flows. Vulnerability to wildfires is increasing as more dwellings are constructed in the Boise foothills adjacent to range and timber lands. During the field trip, examples of flood plain encroachment are illustrated and contrasts between municipal and county hazard mitigation are discussed.