The Boise River below Lucky Peak Dam is flowing at very high levels, especially for March, and water is showing up in some unexpected places. There’s more going on than meets the eye.
Take a drive over one of the bridges or a walk along the Greenbelt and it’s easy to see the river is high and wide and fast.
While the river is confined to a narrow channel by levies in some reaches, there are other areas where the river is still connected to the floodplain and, in those locations, natural flood control processes are at work slowing and temporarily capturing the water. The floodplain on the south side of the river east of Veteran’s Memorial Bridge pictured above is one such location. It’s worth a special trip to view the floodplain from the bridge sidewalk.
In Plain View
There are many places where the river has risen within its confined channel and inundated areas that are normally well above river level, like the Greenbelt on the north bank of the north channel at Eagle Road shown below.
With a bit of sleuthing, you can also discover places where the river has overtopped its bank and is inundating floodplain and wetland areas quite a distance from the normal channel. The photo below was taken west of Boise’s Willow Lane Park on the north side of the river. The Boise Greenbelt was built in the floodway, and, at flows of 7,000 cfs and above, the river overtops the bank and follows its historic path.
Where’s This Water Coming From?
But not all water is moving where we can see it. Why is this parking lot on Riverside Drive in Eagle flooded?
It’s easy to see that the adjacent pond is spilling into the parking lot, but why is the pond so high? There’s no connection between the river and the pond. Or is there?
While we can’t see it, the river and the pond are hydrologically connected under the ground. This development was built in an old channel of the Boise River, and groundwater continues to moves freely through the sand and rocks underneath the buildings and parking lot. When flows increase in the river, the groundwater level – and pond level – rises too. In this case, the water elevation of the river and the pond rose higher than the lowest point of the pond bank and the water spread into low-lying areas of the parking lot.
Many property owners adjacent to the Boise River are finding the river has snuck up on them, traveling through the aquifer into their yards, fields or basements.
The River Didn’t Consult the Blueprints
Why is the Eagle Health Plaza parking lot flooded? This parking lot is about 30 yards from the overfull pond, but there’s no apparent connection between the two. The water from the pond isn’t spilling into this parking lot. My unverified assumption is that the pond and parking lot are connected through a stormwater pipe. Rain and snow melt is expected to flow from the parking lot to the pond, but the river didn’t read the engineering blueprints, and the water is now flowing from the pond uphill and into the parking lot. Another possibility is that there is a stormwater vault under the parking lot that has filled with groundwater and is spilling into the parking lot.
There are likely numerous places where the high and powerful Boise River is now being pushed into stormwater discharge systems. This might cause local inconvenience like at the Eagle Health Plaza or more serious problems during rain storms when stormwater will pond up instead of draining off streets and parking lots.
Lots More To Come This Spring
The Boise River system is loaded with water in the form of mountain snowpack, so it’s likely the Boise River will flow at unusually high levels for months. We’ll all have lots of opportunity to find the Boise River in unexpected places.