Definition
The Missoula floods, also referred to as the Spokane floods or Bretz floods, were a series of massive glacial‑lake outburst floods that occurred during the late Pleistocene epoch, approximately 15,000 to 13,000 years ago. The floods originated from the periodic rupture of an ice dam that contained Glacial Lake Missoula in present‑day western Montana and released vast quantities of meltwater across the Pacific Northwest of the United States.
Overview
Glacial Lake Missoula formed when the Cordilleran Ice Sheet’s Purcell (or Missoula) lobe advanced southward, blocking the Clark Fork River and creating a natural dam near present‑day Missoula, Montana. At its maximum, the lake held an estimated 2,100–2,500 km³ of water, with a depth of up to 600 m. Failure of the ice dam—likely due to buoyancy‑driven lifting or hydraulic pressure—triggered sudden, catastrophic flooding.
The released water surged across the Idaho Panhandle, entered the Columbia River Gorge, and descended the Columbia Plateau, carving the distinctive “Channeled Scablands” of eastern Washington. Geological evidence (giant current ripples, cataract‑sized potholes, dry waterfalls, and enormous boulder deposits) indicates that floodwaters traveled at speeds of 30–60 m s⁻¹, reaching discharge rates of 10–30 million m³ s⁻¹—far exceeding modern river flows.
Research by J. Harlen Bretz in the 1920s and subsequent investigations established that a series of at least 40 such flood events occurred over a span of roughly 2,000 years, each triggered by the reformation and eventual failure of the ice dam after the lake refilled.
Etymology / Origin
The term “Missoula” derives from the Salish word missouli, meaning “people of the river of the big water” or a similar reference to the Missoula River region. The floods are named after Glacial Lake Missoula, the primary source reservoir whose periodic drainage produced the catastrophic events.
Characteristics
| Aspect | Description |
|---|---|
| Source reservoir | Glacial Lake Missoula, confined by an ice dam of the Cordilleran Ice Sheet. |
| Volume per event | Approximately 2,100–2,500 km³ of water released in a single breach. |
| Peak discharge | 10–30 million m³ s⁻¹ (≈10–30 times the Amazon River’s average flow). |
| Velocity | Estimated 30–60 m s⁻¹ (≈108–216 km h⁻¹). |
| Geographic extent | Floodwaters traversed >1,300 km from Montana to the Pacific Ocean, affecting Idaho, Washington, and Oregon. |
| Landscape impact | Formation of the Channeled Scablands, giant current ripples (up to 30 m high), dry falls (e.g., the Dry Falls at the Columbia River), and large basaltic erosion features. |
| Frequency | At least 40 distinct flood events inferred from sedimentary layering and stratigraphic analysis. |
| Chronology | Occurred primarily between ~15,000 and ~13,000 years before present, within the terminal Pleistocene. |
Related Topics
- Glacial Lake Missoula – The proglacial lake whose outbursts generated the floods.
- Cordilleran Ice Sheet – The continental ice sheet that supplied the damming ice lobe.
- Channeled Scablands – The erosional landscape in eastern Washington shaped by the floods.
- J. Harlen Bretz – Geologist who first proposed the flood hypothesis in the 1920s.
- Ice Age Floods National Geologic Trail – A designated trail interpreting the floods’ geological legacy.
- Columbia River Basalt Group – Extensive flood‑basalt flows whose topography interacted with the floodwaters.
- Bonneville Flood – A later Pleistocene outburst flood from Lake Bonneville, often compared with the Missoula floods.
The Missoula floods represent a seminal example of rapid, high‑energy hydrologic processes that have profoundly reshaped the topography of the northwestern United States and continue to inform the fields of geomorphology, paleoclimatology, and Quaternary science.