The City of Johnston is prone to flooding in low lying areas, but only if certain conditions are realized. Two basic types of floods affect our community.

Flash flooding of Beaver Creek is the most common. Comprised of 380 square miles, the Beaver Creek Watershed is in Polk, Dallas, Boone and parts of Greene and Webster counties. When an excessive amount of rain falls in a short amount of time upstream in the Beaver Creek Watershed in Boone and Dallas Counties, this affects Beaver Creek in Johnson. While it affects Johnston, it typically takes 24 to 48 hours before we see a rise in water levels. The water level typically manifests itself as minor flooding of low land areas in the Beaver Creek Natural Resource Area and will sometimes lead to short term closings of the recreational trail system.

When the recreational trails close, the Johnston Parks & Recreation Department will close access gates to the trials. Once the water has receded, the area will be cleaned up and reopened. Officials advise walkers and cyclists to heed the warnings and do not try to cross over any flooded areas.

The second more significant type of flooding occurs when the Des Moines River Basin receives excess rainfall, which forces the Saylorville Reservoir to do an uncontrolled release utilizing the emergency overflow spillway. When this type of release occurs, the already high Des Moines River, South of the Saylorville Dam and at the confluence of the Des Moines River and Beaver Creek will rise. Then the flow contribution from Beaver Creek whether high or normal backs up to the west affecting areas west of the Beaverdale Little League and Prairie Point Crossing Park.

Depending on the flow of the water, low lying areas and businesses along NW Beaver Drive can potentially be affected. Some businesses experienced this in 1993 and 2008, and to a much lesser extent in 2010. There are many more factors that play into this scenario occurring and when Saylorville Reservoir rises, but City staff are in continuous contact with the U.S. Army Corps of Engineers and Polk County Emergency Management.

City staff has gone through flooding in recent years and we use those experiences to better respond by using after action flood summaries. The City of Johnston make every effort to keep residents and business owners informed, should they be in an affected natural disaster area.

The City of Johnston advises residents to sign up for notifications to be notified in the event of a natural disaster or emergency situation.

Quick facts on Des Moines River basin:
  • 14,802 square miles
  • 525 miles long
  • Time it takes water to get to Johnston from headwaters in Minnesota: 7-15 days
  • 1,334 square miles in Minnesota the remainder in Iowa