Sinkholes and Sinkhole Lakes in the PRCF
One of the special features of the Pigeon River Country Forest is its sinkhole lakes.
Sinkholes are an occasional feature in areas where underlying limestone (Fig. 1 below), which is easily dissolved by acid water
- is eroded into what is known as "karst" formations, which often contain underground streams and caves (Fig. 2 below) . Such karst formations are fairly common in the NE portion of Michigan's Lower Peninsula. Sinkholes are formed when the roof of these caves deep underground eventually collapse (Fig. 3).
Some
of these sinkholes are simply deep depressions, sometimes found in a line where an underground stream flows beneath. Such are the dry sinkholes just east of Shoepac Lake a few miles south of Onaway. Others, where the bottom of the sinkhole becomes plugged with marl or other less permeable material, form lakes. Shoepac Lake itself contains several sinkholes, as does Rainy Lake a bit farther to the Northeast, even though, at the surface, these two lakes appear rather ordinary, at least most of the time. (Rainy Lake appears to have lost at least part of its "plug" periodically, and in 1925 went completely dry only to recover within a few years. Between 1980-82 the same lake's level dropped by 45 feet!)Of the eight identified sinkhole lakes in the Pigeon River Country Forest, at least six of them are of the classical round sinkhole type with very steep banks. These six are Lost Lake and West Lost Lake, North and South Twin Lakes, Section Four Lake, and the smallest, known as "the Devil's Soup Bowl". The water of these six is typically milky blue-green reflecting their dissolved limestone origins and are fed mostly by underground springs. Ford Lake looks more like a normal lake with more gentle banks, while Hemlock Lake, while it has the typical steep bank along its eastern and southeastern shore, it has, like Ford Lake, also some more gradual, even swampy shores. The depths of these lakes range from 30 to 60 feet, quite unusual for such small lakes in country with such sandy or gravelly soil. Just southeast of Hemlock Lake is a dry sinkhole similar to those near Shoepac Lake.
All sinkhole lakes in the Pigeon River Country Forest have been closed to fishing for the past four decades while the DNR Fisheries Division conducted studies of the growth and survival rates of various strains of trout. These studies have been finally completed, and the plan is to stock four of these lakes with two of these strains: two (Hemlock Lake and Section 4 Lake) with Assinica strain Brook Trout, and two (Ford Lake and West Lost Lake) with Steelhead strain Rainbow Trout. The three lakes closest to Twin Lakes Rd., because of the danger of increased shore erosion, will not be stocked. But whether stocked with trout or not these sinkhole lakes remain among the most picturesque jewels of the Pigeon River Country Forest.
Section 4 Lake
Text, drawings & photo by R W Kropf 10/12/2006