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South Dakota: Risks & Trends

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Same standard as every other page on this site: every figure below is independently sourced, and figures that vary across sources, or measure genuinely different things, are flagged as such rather than collapsed into one number that looks more precise than the underlying data actually supports.

The 1972 Black Hills flood

MetricFigure
Date June 9-10, 1972
Rainfall near Nemo ~15 in in ~6 hours
Rainfall over 60 sq mi area 10+ in
Trigger Canyon Lake Dam, clogged with debris, failed late on June 9
Deaths 238
Injuries 3,057
Homes destroyed 1,335+
Automobiles destroyed 5,000+
Damage (1972 / 2025-adjusted) $160 million / ~$1.23 billion
Historical ranking The most detrimental flood in South Dakota history and one of the deadliest floods in U.S. history

Rainfall alone didn't cause this; the dam failure did too

Extreme rainfall created the conditions, but the dam failure amplified the disaster into one of the deadliest floods in U.S. history. It’s a reminder that a piece of infrastructure meant to manage water can itself become the point of catastrophic failure once conditions exceed its design.

The 2015 Delmont tornado

MetricFigure
Date May 10, 2015
Rating EF2
Peak winds 130 mph
Path width 400 yards
Path length 16+ mi
Injuries 9
Homes severely damaged/destroyed 44 (12 more damaged)
Power/water outage duration, entire town 2 days
Fire department Destroyed, with hazardous debris scattered through town

Small-town scale, total impact

Delmont’s tornado didn’t approach the death toll of the 1972 flood, but it knocked out an entire town’s power and water for two full days and destroyed the local fire department that would normally respond to exactly this kind of emergency. In a small town, a single storm can disable the whole local response infrastructure at once.

Sources