the railroad and sawmills for processing into lumber. Environment and current use The Clam Lake Canal has been widened six times over the years to about , and as of 2005 is used as a recreational passage between the lakes for local boaters. In 1919, the area was reorganized as Mitchell State Park; the canal itself was dedicated as a Michigan State Historic Site on March 16, 1989. Lake Mitchell, previously known as Big Clam Lake, is about a surface area lake and 95 percent of it is at least deep. Lake Cadillac, previously known as Little Clam Lake, is about a surface area lake and about half of it is deep. They are both fishing lakes, as is the canal, because they have vegetation favorable for a fish environment. Lake Mitchell (Big Clam Lake) reportedly dropped by a foot soon after the canal was completed in 1873. Black Creek still exists and flows in both directions during periods of high water draining the eastern portion of the local marsh into Lake Mitchell and the western portion into Lake Cadillac. The size of the Lake Mitchell drainage basin is over 28,000 acres and the lake flushes about once a year. One report classifies Lake Mitchell as a borderline meso-eutrophic lake and another report classifies it as eutrophic, although some of those parameters show as mesotrophic. The lake is primarily sand and organic matter, with a few areas of cobble stone and gravel. Although there is no dam or lake level control structure on the lake, there is a structure on Lake Cadillac that influences the level of Lake Mitchell. The official lake level for Lake Cadillac (Little Clam Lake) was established in 1967 as an annual maximum level of above sea level. Freezing phenomenon In the winter, Clam Lake Canal often displays an unusual phenomenon, described in Ripley's Believe It Or Not series. Unlike most canals in the Northern Hemisphere, the canal freezes over in the first part of the winter, when the lakes on each side are unfrozen. When the lakes freeze over in the mid-winter months, the canal then thaws and flows with unfrozen water. According to a spokesperson with the Department of Natural Resources, the explanation for this phenomenon concerns the physical properties of water which is heaviest and most dense at , and less dense above and below this temperature. In the early winter months, lighter