An underground chamber of cold rock, engineered to lower the water temperature in a Washington County trout stream, has become the latest defense against contaminated tributaries feeding the lower St. Croix River.

“We’re trying to mimic nature and we’re not super good at it as humans,” said Karen Kill, administrator of the Brown’s Creek Watershed District, speaking of how the “rock crib” maintains a constant temperature year around.

“What we’re trying to do is create a habitat where the cold water species that we would expect to find here will live and thrive.”

Brown’s Creek, which has a state trail by the same name winding alongside it for half of its 8.2-mile length, has a reputation as an impaired waterway. Brown trout struggle in its warmer waters, and too much sand, gravel and other sediments are swept into it by stormwater. Phosphorus contamination that causes algae blooms in the St. Croix remains high in the creek.

On average, Brown’s Creek pours 4.9 million gallons of water into the river daily, the equivalent of about 7½ Olympic swimming pools. Conservation work on Brown’s Creek, Valley Creek and other streams will reduce impairment of Lake St. Croix, the widest portion of the river from Stillwater south to where the St. Croix empties into the Mississippi River at Prescott, Wis.

It was near Brown’s Creek Park in northwest Stillwater, where a downward-sloping Neal Avenue crosses the creek, that a serious threat to water quality persisted for years. Stormwater rushed across a gravel parking lot, spilling sediment and warm water into the creek.

To resolve that problem, the watershed district hired an engineering firm to oversee construction of a grassy drainage area, called a bioswale, to filter sediment from stormwater, and a new parking lot designed to channel water to the bioswale.

And then there’s the 4-foot-deep rock crib, which drains water through perforated pipes onto buried rocks where it will cool by 10 degrees — to a desired 65 degrees Fahrenheit — before being released into Brown’s Creek.

Derek Lash, the environmental engineer directing the project, said rock cribs remain relatively rare, although builders are beginning to use them more in private residential developments. Avoiding any further disturbance to Brown’s Creek was a challenge when building the rock crib, he said.

“I’m ready to see it in use,” Lash said.

Steps toward conservation

The rock crib project is the third major water quality effort on Brown’s Creek in the past year. Last week, Washington County commissioners voted to buy 13 acres of land near the creek’s headwaters in the rural city of Grant with voter-approved state Land and Water Legacy funds. The watershed district will pay half of the $254,400 cost.

In another location a year ago, Washington County and Stillwater agreed to create public parking and protect a stretch of Brown’s Creek and its wetlands on the southeast corner of Hwy. 96 and Manning Avenue.

Conservation agreements will protect both parcels of land against development.

Other watershed districts have taken similar action in recent years to improve water quality and help Lake St. Croix recover from its own impairment, which was serious enough to land that stretch of the river on the Minnesota Pollution Control Agency’s list of impaired waters in 2007.

In south Washington County on Valley Creek, for example, managers took steps to control stormwater runoff and limit erosion. In a northern stretch of the county, the Carnelian-Marine on St. Croix Watershed District repaired a deep ravine where tons of soil had eroded into the St. Croix.

On Brown’s Creek, the rock crib project will help make the difference — at least downstream of the project — between a warm muddy trout stream and a clear cold one. The watershed district received a $204,000 grant for construction, Kill said, and Stillwater will pay for the new parking lot and new curbs and gutters to channel even more stormwater into the filtration.

The watershed district continues to invest in smaller improvements that someday will combine for a larger restoration of a creek, which is highly regarded for its beauty and recreational uses.

To make sure progress is being made, the district monitors the creek at several places. Kill said the news gets better all the time. “We’re starting to see some positive signs. There’s great hope,” she said.