Vegetation Fails the Grade

Thousands of people enjoy floating on the Boise River or a bike ride or walk along its shady shores.  But just how healthy is the Boise River and its shady shore? An examination of the plant communities along the river is one way to measure river health and sustainability.

We can look at a recent report prepared for the City of Boise by the U.S. Army Corps of Engineers (Corps) to find out how healthy the vegetation is. The Boise River Riparian Corridor Stewardship Plan is an inventory of the vegetation cover and ecological condition of the vegetation in a 200-foot setback from the 6,500 cubic feet per second flow line on both sides of the river from Eckert Road to Glenwood Bridge.

The take away from the report is that the vast majority of the vegetation is not providing important ecological services and lacks resilience. The Corps report also includes a plan for management, restoration and sustaining this critical riparian habitat.

Bald Eagle photo: Ken MIracle
Bald Eagle in cottonwood tree. Photo by Ken Miracle.

The importance of healthy vegetation can’t been overstated. According to the U.S. Fish and Wildlife Service, “the typical, and historic, vegetative community adjacent to the Lower Boise River is black cottonwood forest.” According to a woman whose research has been invaluable to me, wildlife biologist and former Fish and Wildlife Service researcher Signe Sather-Blair, “These forested wetlands perform many functions and provide multiple benefits including: groundwater recharge, groundwater discharge, flood storage, reduced flood peaks, increased flow duration, shoreline anchoring, sediment trapping, nutrient retention and removal, food chain support, habitat for fish and wildlife and active and passive human recreation.”

The Corps identified eleven vegetative cover types found in the Boise River floodplain. Three ecological condition classes were used. In Class 1 the overstory and the understory is dominated by native species, historic vegetation composition exists and the plant community is ecologically stable with high resiliency. Soils and dominant ecological processes are fully functioning and plant communities are self sustaining. Class 2 areas have more non-native species and the plant community has only moderate resilience. In Class 3 communities there has been a major departure from historic vegetation composition and the plant community is ecologically unstable with low resilience.

One of the largest areas of Class 1 cover occurs in the reach just below Eckert Bridge.  Large areas of Class 1 mature and immature cottonwood are found in Barber Park; and in Marianne Williams Park and Harris Ranch on the north side of the river, large areas of Class 1 herbaceous wetland are found – much as a result of private and public wetland restoration projects. These areas are publicly accessible, and I recommend taking a walk to see what healthy habitat looks like. More Class 1 cottonwood forest is found downstream of East Park Center Bridge, especially in the new Golda Harris Nature Preserve on the north side.

Healthy vegetation. Photo: Liz Paul
Healthy vegetation across the river. Photo by Liz Paul
With the exception of a few narrow swaths of Class 1 riverside cottonwoods, all of the vegetation is Class 2 or 2- until the Veterans Memorial Parkway Bridge area. A healthy Class 1 area of mature and immature cottonwood trees is found just upstream from the bridge on the south side and mature riverside cottonwoods are found below the bridge on the north (by the Lander St wastewater treatment plant).

Class 1 mature cottonwood forest is found by Mystic Cove Park in Garden City and across the river near the Willow Lane Athletic Complex.  That’s it for Class 1.

Healthy vegetation near the Lander St wastewater treatment plant. Photo: Liz Paul
Healthy vegetation along the Boise River. Photo by Liz Paul.
The Corps recommends a number of maintenance and restoration actions to improve ecological function. That’ll be the topic of my next blog.