by Blair Hurst, Walsh Environmental
|Tour participants walk across historic mined areas adjacent to the Blue River. Note height and extent of tailings piles.
As a part of the 2007 Sustaining Colorado Watersheds Conference held in the Town of Breckenridge, engineer Peggy Bailey of Tetra Tech, Inc., led a bicycle tour of dredge-mining impacted and restored sites along the Blue River. Tour participants enjoyed an informative bike ride on a 5-mile stretch of the Blue River Recreation Path, under a bright blue sky amidst golden aspen leaves. Several stops along the way allowed us time to examine restored sites, active mine sites and an historic mine site.
Dredge mining, a form of placer mining, employed large boats 100-feet in length to excavate river alluvium to depths of 60 to 90 feet. A typical boat had over forty 9.5-foot buckets that when in full operation could dredge 2,500 cubic yards of gravel and dirt every 24 hours. Excavated alluvia are sorted manually, gold is removed, and large tailings piles are deposited behind the boat.
|View of Blue River along a 1.5-mile stretch of the Blue River Restoration project.
Dredge boat mining is considered a financial cornerstone of the Breckenridge mining heritage. However, this destructive land-use practice has left a legacy of negative environmental consequences. The dredges literally turned the riverbed upside-down, removing vegetation and pushing fine soils to the depths below or sending them downstream in the water column. The extent of disturbance along the Blue River encompasses 5 miles of stream corridor that in places extends up to 1,200 feet in width. One stop on our bicycle tour at a remaining non-restored historic mining site exemplified the results of dredging. Here the channel is linear, trapezoidal, and devoid of vegetation. Before restoration efforts in the locations described below, groundwater levels reportedly fluctuated up to 20 feet, low flows channeled underground and downstream through the voids in the dredged alluvium, and aquatic habitat was virtually non-existent.
Bailey described three restoration projects undertaken over the past 20-years. All projects are multi-disciplinary, requiring hydrologic, hydraulic and sediment transport analyses, a vegetation component, and consideration of habitat, land use and property ownership issues.
The Blue River Restoration Master Plan provides guidance for a 2.5-mile restoration project in an area north of Breckenridge. Goals of the plan include restoring continuous year-round flows (surfacing) and natural fluvial processes, enhancing the riparian zone, and designing stable banks. Intensive data collection and analyses were necessary and included mapping and measuring cross sections, measuring flows, monitoring groundwater levels, creating vegetation and habitat inventories, and performing geomorphic evaluations and sediment transport analyses.
A second project, the Blue River Restoration, restored a 1.5-mile reach on the north end within Breckenridge. Dredge rock along this stretch was 10 to 30 feet deep and low flows disappeared into the voids between coarse sediment. Results from this project include 10 acres of stable wetlands restored with native vegetation and enhanced recreational opportunities, including 6,000 linear feet of new and improved fish habitat.
The third project Bailey described is the Blue River Walkway project. This project comprises a one-mile reach located in downtown Breckenridge. Due to its location in the center of downtown, this project presented challenges far beyond that of surfacing the river. Confinement of the 100-year flood was paramount. Given the proximity of existing built structures, utilities had to be relocated and pedestrian and vehicle crossings had to be considered. In spite of such challenges, the Blue River Walkway project successfully combines native plants with urban landscaping to improve 5,400 linear feet of riparian corridor, enhances aesthetics and recreation in this urban setting, and provides the requisite flood control.
The tour concluded in downtown Breckenridge at the Dredge Restaurant, next to the completed Blue River Walkway. The Dredge Restaurant, a 2-million pound replica of one of the largest gold dredge boats to operate in Breckenridge, floats on a pond in the middle of town. From this precise vantage point, one could simultaneously imagine a time when gold dredge mining dominated this reach of the Blue River while seeing present improvements made in the wake of this rich heritage.