You know that heavy rain can cause flooding, but did you know that snow and ice can also cause flooding?
When temperatures start to warm each spring, the threat of flooding grows across the part of the country that spent the winter in a deep freeze. In northern U.S. states like Alaska, North Dakota and Wisconsin, and also in the mountains of the Western U.S., temperatures during the winter mostly stay below freezing. This means all the snow that falls in this area hangs around until it is warm enough to melt. Once it does start to melt, the run-off from the snowmelt will head for nearby creeks, rivers, lakes and reservoirs, and may cause these bodies of water to overflow their banks.
It is possible for that same flood to affect areas well downstream of where the flooding originated.
For example, almost every spring flooding on the Mississippi River starts in the Upper Midwest as the temperatures warm and the snow and ice start to melt. All that additional water causes the Mississippi River to spill over its banks in the north. As this excess of water travels down the river towards the Gulf of Mexico, many areas that didn’t receive much snowfall over the winter still experience the flooding that is happening due to the snowfall in the north.
Thick ice also forms on some rivers and creeks during the winter. When the temperatures start to warm, the ice starts to melt and it weakens enough to crack and break apart into chunks. As ice is carried downstream, it may get caught on any sort of obstruction to the water flow, such as a downed tree, further blocking the water’s ability to flow. When this occurs, water can be held back, causing upstream flooding. When the jam finally breaks, typically because of further ice melting, flooding can occur downstream.
Ice only six inches thick can destroy large trees and knock houses off their foundations. Once an ice jam gives way, a location may experience a flash flood as all the water and debris that were trapped rush downstream. This is why it is important to know when a nearby river is impacted by an ice jam and to avoid the area until the ice jam has been resolved.
The National Weather Service provides river forecasts so that you will know when a river might flood and can stay safe by avoiding that area. Visit www.water.weather.gov for the latest river forecast information.
This flooding in downtown Davenport, Iowa, from the Mississippi River was caused by snowmelt.
