Snow becomes water; water becomes flood risk
Snow gives us skiing in the winter and rafting in the summer. It is a critical element of the water cycle; as much as 75 percent of the water supply in the western United States comes from melting snow. It helps drive the spring flows in the Boise River and can provide the total flow for smaller, unregulated streams and creeks that flow in and around the community. These flows vary from year to year depending on the amount of snow in the mountains and the spring weather.
From flow to flood
Most years in the Treasure Valley, the relationship between the snowpack and the spring weather is a good one. Runoff flows are handled by the reservoirs, rivers and creeks without causing major flooding. But these favorable conditions do not exist every year. The following factors can help create snow melt flooding:
Very moist soil prior to the snow melt: Fall rains can soak the soil and cool weather can keep the ground from drying out. This limits how much moisture the ground can absorb.
Frozen soil or ground frost: Frozen ground will prevent water infiltration into the soil.
Heavy winter snow cover: More snow equals more water. If the heavy snow is widespread, it will keep the air cool and the snow may stay later in the year. This snow is then susceptible to the next two issues.
Rain-on-snow events: Widespread rain during the snow melt will warm up the snowpack and increase the flow to rivers and streams. The combination of rain and snow melt can cause flash flooding.
Rapid snow melt: Snow melt rates are normally similar to a light to moderate rainfall. However, a sudden warming trend with nighttime temperatures above freezing can create much higher melt rates.
In addition to flooding, rapid snow melt can cause landslides, debris flows and contribute to ice jams.
Take the first step toward being prepared … be aware
The Internet has made it much easier for all of us to access information that was once only available to professional forecasters. The following sites will provide useful information on snow melt.
Idaho SNOTEL Snow/Water Equivalent
National Weather Service Hydrological Prediction Service
These Websites will allow to you monitor the snowpack in the mountains and the river flows in the valleys. Keeping a “weather eye” on your environment will help you see a potential disaster before it arrives.
You can better develop your “weather eye” by learning to be a Weather Spotter (follow the link). The next class in Ada County will be:
When: Wednesday, April 25, 2012 from 7 p.m. to to 9 p.m.
Where: Public Safety Building, 7200 Barrister Drive, Boise. Meet in the Lobby no later than 7pm.