Water will go downhill the fastest and easiest way possible. It will wear away stone and carry away soil, undercut trees and carve deep gullies, just to find its way to sea level.
Given enough time, gravity and water can overcome almost any obstacle.
It’s a perfectly natural process, but when more water than normal is sent down a St. Croix Valley bluff, it can cut a path that threatens the health of the river below. By washing soil into the river, runoff muddies the water and fuels the growth of harmful algae.
As reported last month, Chisago County has stabilized several gullies along its river bluffs the past few years.
A little ways south, the local watershed district also recently restored a bluff on the border of Scandia and Marine on St. Croix, overcoming difficult conditions to fix a serious source of sediment in the St. Croix River.
A growing gully
The gully just east of Highway 95 in the historic area of Copas had been a problem for almost ten years. It slashed 80 feet down through the bluff at a 55 percent slope. Tons of soil and nutrients were eroding from the bluff, suffocating a wetland below and dirtying the St. Croix River a short distance beyond.
A storm sewer pipe jutted out at the top of the bluff, and during a rainstorm water poured out of it. In a major storm, it could flow at more than 1,000 gallons per minute.
The ravine got broader and deeper with every downpour. By 2015, it was thirty feet across and fifteen feet deep, widening by four to five feet per year.
That’s when the Carnelian-Marine-St. Croix Watershed District, environmental engineers, neighbors, and some bold excavator operators came to the rescue.
The watershed district led the effort to restore the bluff and re-route the water. When watershed district managers and others recently visited the site to celebrate the success, there was no indication of the muddy mess they would have seen this time last year.
Not so simple
The solution involved a massive plastic pipe, replanting the slope with native vegetation that would hold the soil in place, installing a rain garden to let runoff soak in before getting to the bluff, and removing some trees to let more light through to the ground.
That sounds relatively simple, but it was a big challenge.
Paying for the project presented the Carnelian-Marine-St. Croix Watershed District with its first hurdle.
“As always, funding was a big issue,” says district administrator Jim Shaver. “We included this project in several unsuccessful grant applications, looked to Scandia to share costs, but eventually felt that the issue was critical enough to fund on our own.”
The watershed district receives tax dollars from local residents, which it used to pay for the $75,000 project. With the gully only getting bigger and badder every year, they couldn’t wait for outside dollars.
Steep and slippery
They hired Wenck Associates to design the restoration, and Blackstone Contractors to make it reality. The site was a special challenge because of the “severe slope” and bedrock outcroppings. Wetlands surrounding the site also made it difficult to access.
That was in good conditions. The construction crew did not have good conditions.
“The weather in November of last year changed while we were waiting for the availability of a critical subcontractor, forcing the equipment operators to work on the wet and sometimes snow-covered slope,” Shaver says.
The primary excavator operator had once been a logger in the Pacific Northwest, and knew how to handle the massive machine on the steep and slick terrain.
Removing up to two feet of sediment from the spring-fed black ash swamp at the foot of the bluff, and planting it with native plants well-suited to the site, the crew restored what Shaver calls a “highly valued resource type.”
They also filled in the gully except for a trench down the middle, where a pipe was placed.
Not just any pipe. By welding 12″ plastic pipes together, a massive 130′ pipe was constructed at the top of the ravine, and then laid into the trench. It now carries runoff water down the bluff, without washing away any soil.
The phosphorus the project is keeping out of the St. Croix is enough to fuel the growth of about six tons of algae. While algae is also natural, it can cause big, toxic blooms when it gets too much phosphorus.
Altogether, the 197th St. gully repairs will keep 33 tons of sediment and 43 lbs of phosphorus out of the river each year. That’s a one-time cost of about $1.13 per pound of soil kept on the bluff – and the benefit to the river is repeated each year.
When the district managers visited in mid-October, the hillside was still green with grass and dotted with wildflowers. Next year, the watershed district hopes to continue improving the site, working with the city of Scandia as other road work is done nearby.
From the top, the slope looked almost vertical. Bare hills on the Wisconsin side were visible across the valley. The bluff looked like it would be there another thousand years.