The Missoula Floods: Earth’s Real Megafloods That Happened Dozens of Times

The Missoula Floods (also called the Spokane Floods or Bretz Floods) were a series of catastrophic glacial lake outburst floods (jökulhlaups) at the end of the last Ice Age.

They repeatedly inundated parts of western Montana, northern Idaho, eastern Washington, and northern Oregon, flowing down the Columbia River to the Pacific.

During the late Pleistocene (roughly 20,000- 14,000 years ago), a lobe of the Cordilleran Ice Sheet (the Purcell Trench lobe) advanced south into the Idaho Panhandle near present- day Lake Pend Oreille. It dammed the Clark Fork River with an ice barrier up to about 1 km (roughly 2,000- 3,000 and more feet) thick.

Meltwater backed up behind this dam, forming Glacial Lake Missoula in the valleys of western Montana. At its maximum, the lake covered roughly 3,000 square miles (about 7,800 km²), reached depths of ~2,000 feet (600 and more m) at the dam, and held an estimated 500- 530 cubic miles (roughly 2,100–2,200 km³) of water, comparable to the combined volume of Lakes Erie and Ontario.

When the water deepened enough, the ice dam failed (likely via flotation, subglacial tunneling, or related processes). The lake drained catastrophically in a matter of days. The ice dam then re- formed, the lake refilled over decades, and the cycle repeated dozens (likely 40 to more than 100) of times over a period of about 2,000- 4,000 years (primarily ~20- 14 ka).

Peak discharges for the largest floods are estimated in the range of several million to over 10- 17 million cubic meters per second (orders of magnitude larger than the Amazon River’s flow). Water speeds reached tens of meters per second (up to ~50- 80 and more mph in places), with walls of water potentially hundreds of feet high.

The floods:

• Scoured the Channeled Scablands of eastern Washington, an anastomosing network of coulees, dry cataracts, and scoured basalt over ~30,000 km².
• Carved features like the Grand Coulee, Dry Falls, and parts of the Columbia River Gorge.
• Deposited giant gravel bars, current ripples, ice- rafted erratics (including car-sized boulders carried hundreds of miles), and thick slack water sediments (rhythmites).
• Temporarily ponded large temporary lakes (e.g., Lake Lewis in the Pasco Basin) and back- flooded the Willamette Valley in Oregon.
• Transported sediment far downstream into the Pacific, with deposits on the abyssal plain.⁠

Early floods often followed the Columbia valley route; later ones were diverted southward into the scablands after the Okanogan lobe blocked the northern path and formed Glacial Lake Columbia

In the 1920s, geologist J Harlen Bretz proposed that the unusual landscape of the Channeled Scablands resulted from a single enormous flood (“the Spokane Flood”).

This was highly controversial because it challenged gradualist views. Joseph T. Pardee later identified Glacial Lake Missoula as the source (via shoreline evidence and giant current ripples).

Multiple floods became widely accepted through stratigraphic evidence (rhythmites, intervening soils, ash layers, varves) in the mid- to- late 20th century. Modern reviews (e.g., O’Connor, Baker, and others, 2020) synthesize the evidence for dozens to over a hundred events.

The multiple flood hypothesis for the Missoula Floods holds that Glacial Lake Missoula did not empty in a single cataclysmic event. Instead, the ice dam failed repeatedly, producing dozens (and likely more than a hundred) separate outburst floods over several thousand years near the end of the last Ice Age.

J Harlen Bretz originally proposed a single enormous “Spokane Flood” in the 1920s to explain the Channeled Scablands. By the 1950s, evidence (including Pardee’s work on Lake Missoula) supported catastrophic flooding, and Bretz himself later recognized multiple events.

The modern multiple- flood view was formalized by Richard B. Waitt Jr. in 1980 (often called the “forty floods” hypothesis), with strong support from Brian Atwater and later workers.

The strongest support comes from stratigraphy:

• Rhythmites (Touchet Beds and similar deposits): In slack water areas (e.g., Burlingame Canyon in the Walla Walla Valley and other back- flooded valleys), sequences of 40 or more graded sand- to- silt beds occur. Each rhythmite is interpreted as the deposit of one flood. Intervening layers of windblown dust (loess), soil development, mud cracks, animal burrows, and volcanic ash (tephra) indicate significant time gaps between events rather than continuous deposition from a single flood.⁠
• Mount St. Helens tephra: A particularly compelling example involves two layers of Mount St. Helens ash separated by thin non- volcanic silt between successive rhythmites, demonstrating that years to decades passed between floods.
• Varves in glacial lake deposits: In former Glacial Lake Missoula and Glacial Lake Columbia basins (e.g., Ninemile Creek, Sanpoil Valley), packages of 20– 55 annual varves (typically averaging ~30- 40) overlie individual flood beds. These record lake refilling periods of decades between outbursts.
• Other indicators: Disconformities, paleomagnetic data, radiocarbon ages, and offshore sediment packages at the mouth of the Columbia River (thick sequences deposited over millennia) align with repeated events.

Waitt estimated ~40 floods with average recurrence of ~40- 55 years (later refined to intervals possibly as long as ~100 years early on, shortening to just a few years near the end). Atwater’s work in northern areas suggested even higher counts in some sections (approaching 80- 90 or more in places).

A comprehensive synthesis (O’Connor, Baker, Waitt and colleagues) concludes there were dozens, likely more than a hundred, separate Missoula floods between roughly 20- 14 ka (mainly concentrated ~18.5- 15 ka). Roughly ~75 floods preceded the ~16 ka Mount St. Helens Set S tephra, with 30 or more afterward.

Not every flood was equally large; the biggest ones had peak discharges exceeding 10 million m³/s, while many were smaller. Routing also changed as ice lobes advanced and retreated (e.g., diversion into the scablands after the Okanogan lobe blocked the northern Columbia valley).

The ice dam repeatedly reformed after each failure because the Cordilleran Ice Sheet remained active. Lake Missoula would refill until hydrostatic pressure (or related processes such as tunneling) caused another breach.

While a minority of researchers have occasionally argued for fewer (or even single) major events, the overwhelming stratigraphic, sedimentologic, and chronological evidence supports the multiple- flood interpretation. This framework reconciles the catastrophic landforms with uniformitarian principles by showing repeated, self- dumping jökulhlaups over a multi-millennial period.

“The Great Flood” most commonly refers to the biblical flood narrative (Noah’s Flood in Genesis 6- 9), in which God floods the entire Earth to wipe out humanity (except Noah, his family, and the animals on the ark). Similar flood myths appear in many cultures, including the much older Mesopotamian accounts (Epic of Gilgamesh, Atrahasis, and Sumerian stories of Ziusudra and Utnapishtim).

There is no geological evidence for a single global flood that covered all the Earth’s mountains within the last several thousand years (or at any time in human history).

Key reasons include:

• There is not enough water on Earth (in oceans, ice, groundwater, and atmosphere combined) to cover the highest mountains.
• The rock record, ice cores (Greenland and Antarctica), tree rings, lake sediments, and continuous cultural records (e.g., Egyptian and Mesopotamian civilizations) show no interruption consistent with a worldwide inundation.
• Stratigraphy shows gradual deposition, multiple periods of erosion, soil formation, and distinct fossil sequences over vast timescales, not a single chaotic global layer.

Flood geology (the young- Earth creationist attempts to explain most of the geologic column via Noah’s Flood) is rejected by the scientific consensus in geology, paleontology, and related fields as inconsistent with the evidence.

Regional Catastrophic Floods and Possible Myth Origins

Many cultures have flood stories, which are often explained as memories of real, large local or regional disasters. Candidates that have been proposed (with varying degrees of support) include:

• Post- glacial sea- level rise (~20,000- 7,000 years ago): Global sea level rose ~120 meters as Ice Age glaciers melted, flooding vast coastal plains and forcing human migrations worldwide.
• Black Sea deluge hypothesis (Ryan & Pitman, 1990s): Mediterranean seawater catastrophically flooded a lower- level freshwater Black Sea lake around 7,500-8,400 years ago. This is controversial; many researchers argue the reconnection was more gradual, with less dramatic impact. Some have speculated it could underlie Near Eastern flood myths, though the link remains unproven.⁠
• Other regional events: Catastrophic flooding of the Persian Gulf region, Chinese “Great Flood” along the Yellow River (~1920 BCE, linked to the legendary Xia dynasty and Emperor Yu), the Storegga tsunami (~8,200 years ago off Norway), and various glacial lake outburst floods.

Relation to the Missoula Floods

The Missoula Floods (the subject of our earlier discussion) were real, repeated mega floods in the Pacific Northwest ~20,000- 14,000 years ago.

They were enormous by any standard (peak discharges many times the combined flow of all modern rivers) and left dramatic landscape features, but they were strictly regional, confined to parts of Montana, Idaho, Washington, and Oregon. They have no direct connection to the biblical narrative or Near Eastern flood myths, which are geographically and culturally distant. People were present in the Americas by then and may have witnessed the later Missoula events, but these floods do not match the global description in Genesis.

In short, the Missoula Floods are one of the best- documented Ice Age mega flood systems on Earth. The biblical “Great Flood” is a powerful cultural and religious story shared across traditions, but it does not correspond to a single global geological event.

Real catastrophic floods have occurred many times in Earth’s history (especially during deglaciations), and some of them likely inspired local flood legends.