We at Crossroads at Big Creek are preparing for the sucker run, so it may seem odd that our family programs this week will feature rocks and geology. The connection, however, is direct. Without the Ice Age glaciers, the Great Lakes and their freshwater fisheries would not exist.

Rocks and fossils also play an important role in fish spawning in our creeks. Very soon, fish will leave the big lake and swim upstream because tributaries provide optimal breeding habitat. While northern pike move through streams on their way to warm, shallow wetlands and even flooded fields with emergent vegetation, suckers (and trout) favor gravel-bottomed creeks.

The reason the creeks contain so much gravel goes back to the glaciers. During the last Ice Age, massive ice sheets moved south from the north, crushing mountains and gouging out bedrock. The weight and movement of the ice ground rock into smaller pieces. When the glaciers melted, they deposited what geologists call “till,” a mixture of clay, sand, gravel, and pebbles.

Since the last glacial retreat, much of the finer silt and sand in local tributaries has gradually been washed downstream, leaving behind harder materials such as dolomite—often containing fossils—along with chert and igneous rocks carried south by the glaciers. Over many years, flowing water and weather have rounded these stones into pebbles.

These pebbles create ideal spawning gravel. Fish eggs slip into the spaces between the stones, where they are protected from many predators and are less likely to be smothered by silt. For spawning fish and their developing young, these rocky streambeds greatly increase survival, making tributary creeks especially valuable nursery habitat.

Fish benefit greatly from these rock-filled tributaries, but they also give back, enriching the ecosystems that sustain them.

As fish move upstream, they transfer nutrients—especially nitrogen and phosphorus—from the lake into the stream. These nutrients enter the system through eggs and milt as well as through natural waste.

Through their foraging and energetic spawning activity, fish also stir up nutrients from the sediment and release them into the water, where they become available to aquatic plants and algae that form the base of the stream’s food web. Other wildlife feed on fish eggs, fry, and spawning adults, further linking these seasonal migrations to the broader ecosystem.

So there really will be something a little fishy about our Nature Investigators Rock Festival at 3:30 pm on Thursday and the Saturday Science Fossil Hunts at 10:00 am.

And while it is early for suckers, this is about the time we begin seeing northern pike moving through Big Creek on their way to mucky wetlands, ditches and flooded fields. Our trails along the creek and throughout the preserve are open and free of charge all day every day.

– by Coggin Heeringa, Interpretive Naturalist

For more information, please check out our Crossroads calendar or stop by the Collins Learning Center and pick up a printed calendar. Programs are free and our trails are open all day every day, free of charge, thanks to donors like you!

Crossroads at Big Creek Learning Center and Nature Preserve is located at 2041 Michigan Street, Sturgeon Bay. Crossroads is a 501(c)(3) non-profit NGO committed to inspiring stewardship of the environment through education, research, and outdoor experiences. We welcome your support. 

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Upcoming Activities

Wandering Wednesday – Naturalist Led Hike

Nature Investigators – Rock Festival

Science Saturdays – Fossil Hunt

Wandering Wednesday – Naturalist Led Hike