Courtesy Photo A white heron spreads its wings in the Alpena Wildlife Sanctuary in this photo provided by the Thunder Bay River Center.

Thunder Bay River watershed is a system of lakes, streams, rivers, reservoirs, and wetlands — and all the underlying groundwater, which drains into the river and eventually intoLake Huron. But the watershed is more than that; it includes all the land within its drainage system. Its landscape includes forests, fields, towns, villages, farms and especially the bedrock upon which the land rests. The watershed is a really big deal!

In our watershed, there is a unique type of bedrock called limestone. As stone goes, limestone is a soft rock which can be washed away by water. When the limestone erodes (breaks down) it creates very interesting and some valuable formations like caves, sinkholes, underground streams and lakes. You’ve seen them for sure: the sinkholes near Posen, the large limestone pits outside of Alpena and Rogers City. This type of bedrock is called karst. Karst has proven to be a valuable economic resource for businesses in Northeast Michigan, like LaFarge and for eco-tourist opportunities which support many of our small businesses.

When you’re out for a walk next time, keep looking down. Often, you’ll find small surfaces of limestone or karst peeking out from under the soil. One of the results of the karst typography is Sunken Lake near Posen. Although the source of the North Branch of the Thunder Bay River is in Rush Lake, much further upstream, Sunken Lake is an important part of the river’s north branch. Sunken Lake formed when the karst below it eroded away, the land sank and filled with water. It’s a great place to relax and have fun. Sunken Lake Park, which surrounds the lake, is 160 acres including a campground, many trails and, of course, excellent fishing.

Nearby Mystery Valley is a unit of the Thunder Bay Karst Preserve, along with Stevens Twins Sinks and Bruski Sink. The 76-acre Mystery Valley Karst Preserve and Nature Sanctuary is located in Presque Isle County just a few miles north of the Thunder Bay Karst Preserve. It contains one of the largest karst “collapse valleys” in the Great Lakes region, several dramatic earth cracks and a lake that rises and falls, and sometimes disappears! Visitors to the preserve can follow two self-guided trails: Earthcrack Trail and Valley Trail. Earthcrack Trail passes a series of cracks, including two that converge into one that is several hundred feet long and nearly 15 feet deep.

Following the Valley Trail, visitors can see fossils of marine invertebrates such as brachiopods, bryzoa and crinoids that lived some 350 million to 400 million years ago. Unlike a valley carved by a river, Mystery Valley was formed by the collapse of the surface into a labyrinth of subterranean chambers created by the water erosion of rock below. Mystery Valley is 1.5 miles long, 500 yards wide at its widest point and about 150 feet deep, making it one of the largest known collapse valleys in the Great Lakes region.

We are lucky to live in the Thunder Bay River Watershed. It is unique and uniquely ours to protect and preserve.

The author is an executive committee member for the Thunder Bay River Center. To receive further information, to volunteer, or to make a donation, go to thunderbayrivercenter.org, or visit the Thunder Bay River Center Facebook page.