Dredge boats mined placer gold deposits on the Swan and Blue Rivers in Summit County from 1898 until 1942. Early boats moved 1,000 cubic yards of gravel each and every 24 hours. The boats were 96 feet by 30 feet and had engines with 100 horsepower. A bucket line moved material to the rear of the dredge, dumping it on a revolving "grizzly" and Trommel screen. Larger stones went into a stacker with 25 pockets about 2 feet square. The stacker piled the stones behind the boat, creating the long piles along the river. The fines washed out through holes in the side of the grizzly into riffles on both sides.
Larger boats were soon built as the first boats were deemed too small. The hull of the fourth dredge, which operated from about 1900 to 1905, still floats in its last pond on the Swan River near Galena Gulch. Now the trout in the pond use the remains for shade. This boat chewed through the stream day and night, no doubt to the detriment of those trying to sleep nearby. Known as the Risdon, the dredge produced $20,000 worth of gold in the 1904 season, while moving 2,500 cubic yards of gravel a day.
By 1905, a very large boat was operating in French Gulch, with a capacity of 3,000 cubic yards every 24 hours. In 1906, a boat known as Reliance claimed production of $50,000 in gold in seven months. In 1908, it was converted from steam to electric power. In 1909, Reliance hit a "hot spot" in French Gulch. $40,000 in gold was cleaned from an area less than half an acre in size. Two dredges were operating in French Gulch now, and they produced 21,902 ounces of gold in 1909, and 17,839 ounces in 1910.
The town of Valdora was the service center for the dredge operations. I've been to the site but there is virtually nothing left indicating habitation. Valdora was on the east bank of the Swan, just upstream from its confluence with the Blue River. The town site was destroyed by a dredge boat after the inhabitants moved away.
Nine different boats mined the Brekenridge area. The last dredge worked the Blue River off and on from 1926 until 1942. Then Order L-203 of the War Production Board stopped all mining in Colorado, as war production took precedence over gold production.