The 2011 Souris River Flood: Will It Happen Again?

Minot, North Dakota, in the 2011 flood. USGS.

The bottom line

Flood risk in the Souris River Basin will stay high until its current wet climate state ends. While it lasts:

The 2011 flood

• The basin: 61,000 square kilometers in Saskatchewan, Manitoba and North Dakota.
• The rain: record rains in May and June 2011.
• The peak: 762 cubic meters per second at the USGS gage above Minot, N. Dak. — more than twice any earlier recorded peak and more than five times the estimated 100-year flow since regulation (142).
• The damage: in Minot, more than 4,000 homes flooded and 11,000 people were evacuated.

The Souris River Basin and its neighbors. USGS.

Upstream of Minot the river is regulated by three reservoirs in Saskatchewan and one in North Dakota:

The four reservoirs on June 23, 2011: Rafferty (A), Boundary (B), Alameda (C) and Lake Darling (D). USGS.

The Souris River in flood, 2011. USGS.

Building the model

The Souris River Basin Task Force asked the USGS, with the North Dakota State Water Commission, to build a model estimating the odds of future floods and droughts. It had four parts.

1. Past climate

Tree rings, analyzed statistically, stretched precipitation records back to the early 1700s — "hindcasting" — using five clusters of weather stations with similar precipitation.

• Seasons: runoff depends heavily on season, so the year was split into three 4-month seasons: November–February, March–June and July–October.
• A shift: in cluster 5, summer and fall precipitation stepped up in the late 1970s, matching earlier studies.

The five clusters of weather stations used in the tree-ring analysis. USGS.

Modeled (tree-ring) and observed 12-year average precipitation for southern Manitoba and southeastern Saskatchewan, July–October. USGS.

2. Future climate

The extended records and measured weather data fed a stochastic climate model — a statistical simulator of precipitation, temperature and potential evapotranspiration (evaporation plus plants' transpiration).

It found a step trend around 1970: 1912–69 was a dry climate state and 1970–2011 a wet one — matching nearby Devils Lake Basin, which also alternates between wet and dry.

3. Streamflow

A water-balance model turned simulated climate into monthly natural streamflow — flow as it would be without the four reservoirs. It tracks water coming in (rain and snow) and going out (runoff, subsurface flow and evapotranspiration). A routing model then moved the flows through the basin.

How the water-balance model works. USGS.

4. Will it flood again?

Combining the climate model with the basin-response models gave a stochastic streamflow model. It generated 100 possible futures of 100 years each — 10,000 simulated years.

One simulated century below Rafferty Reservoir: 50 wet years, then 50 dry. USGS.

Two sets of odds: flood-frequency curves normally assume the odds are the same every year. The Souris basin's alternating climate means there are two equilibrium curves — one for the wet state, one for the dry.

At the site representing inflows to Rafferty Reservoir, what matters is each year's total volume rather than the instantaneous peak. In 2011 the inflow was about twice the reservoir's 633 million cubic meters of storage; spillways ran at full capacity and the reservoir could no longer control outflow. That gives the odds in the table above.

Annual inflow volume by exceedance probability, wet (1970–2011) and dry (1912–69) states; 2011 stands out above Rafferty's capacity. USGS.

When will it end? Past data say the wet state will give way to a dry one — but when is uncertain.

Sources

Based on 2011 Souris River Flood — Will It Happen Again?, USGS Fact Sheet 2016–3073, prepared with the North Dakota State Water Commission, U.S. Geological Survey (DOI), summarizing Kolars and others (2016, USGS Scientific Investigations Report 2015–5185) and Ryberg and others (2016), and citing Garbrecht and Rossel (2002), Shapely and others (2005), Vecchia (2008) and Small and Islam (2008); a work of the United States government in the public domain. Figures and photographs come from the fact sheet's PDF; its screenshot of an online story map is left out.