Risk Factors Associated with Floods

Risk Factors

Living Within a Floodplain

If you live in an area that naturally floods during the spring when runoff is high, or during the end of the winter when ice jams form, you are at a higher risk for flooding. To see what your risk for flooding is, check out FEMA's Map Service Center for more information. If you would prefer to look at the most currently available maps in person, you can contact your local floodplain administrator.

If you live in the Town of Jackson you can contact the Engineering Department (your floodplain administrator) to see FEMA Flood Insurance Rate Maps (FIRM's), which calculate your risk of flooding. If you live out in the County, Teton County Engineer's Office would be your floodplain administrator.

Stalled Storm Systems With Heavy Rain

Just as with flash floods, a storm system that is slow moving with high precipitation rates can easily cause flooding. In areas such as narrow canyons this can result in flash flooding. In open areas along rivers, lakes, or streams this can cause flooding that starts slowly but also takes a long time to recede.

Sudden Extreme Temperature Drops Near Moving Bodies of Water

When the surfaces of creeks and rivers begin to lose heat at high rates (like when a sudden cold front moves through), this can cause water to become super cooled. The turbulence from the water flow then causes this super cooled water to mix throughout the entire body of the water's depth. This creates what is known as "frazil ice". Frazil ice tends to not be as buoyant as regular ice, so it begins to accumulate on the upstream side of rocks on the stream bed. This freezing from the bottom-up of the stream or river reduces the volume available for water to flow, causing it to top the banks. This frazil ice can also catch pieces of surface ice causing ice jams.

Ice jams are essentially dams made up of frazil ice and pieces of surface ice. They can occur any time during the winter, but we usually see them in late winter or very early spring when surface ice begins breaking up and gets caught up on frazil ice as it flows downstream. These ice jams can be dangerous for two reasons:

Snow in Hand
Icy Lake

Local Activity

Locally, we typically get ice jams on areas of Flat Creek around N Highway 89 (the Dairy Queen) and also further downstream near the Southern end of South Park Loop Rd in the Melody Ranch area. It isn't beyond the realm of possibility for ice jams to form on the Snake River, but in the last 30 years there have only been small ice floes and light frazil ice formation on the Snake. Needless to say, an ice jam on the Snake River would have much more dire consequences than on Flat Creek.

  • First, the ice jams can cause flooding upstream as they prevent water from flowing downstream.
  • Second, if the ice jam suddenly breaks, it can release all of the water it was holding back, causing flash flooding downstream.

A Winter With High Snowfall Amounts Followed by a Rapid Spring Thaw

A rapid spring thaw can cause increased rates of snow melt resulting in flooding. The degree of springtime flooding potential is determined by the amount of remaining snow pack, the spring temperature trend, and the capacity of streams, rivers, and lakes to handle the snow melt. You can check the National Weather Service web site for current hydro-logic data for our area.

A "Rain-on-Snow" Event

Snow can act like a tarp, preventing rain water from reaching our loose, gravelly soils to be absorbed before it runs into streams and creeks.  When we have early spring rains that fall on snow, it can accelerate the rise in creeks, streams, and rivers more than if that same amount of precipitation fell on exposed soil.