Mill Creek under Cover

It's been a hard winter. The Aberfoyle Reach froze early this year, a bit before Christmas and even with January's mini-thaw has remained ice covered. The frozen creek has become a corridor to get out and explore the winter landscape. 

Even in the deepest winter, some of the stream is open, particularly in fast flowing shallower reaches and in areas of  significant groundwater input. Groundwater, at a constant 8 ^oC  throughout the year,  is significantly warmer than the creek.  While open reaches help to maintain high dissolved oxygen concentrations throughout the stream, they also have their drawbacks. Air temperatures below -6 ^oC can lead to the formation of frazil ice, a Slushee-like mixture of ice and supercooled water that behaves a lot like lava or flowing cement. Under the wrong conditions, frazil ice can build up into anchor ice, blocking channels and scouring the stream bottom.

Beneath the ice, life continues. Under the gravel, brook and brown trout eggs, deposited in redds dug last autumn, are slowly developing into fry which will emerge with spring's freshets.  Groundwater upwelling through the gravel helps protect the eggs from  freezing and provides a refuge for adult and juvenile fish, when deep pools in main channels may become unsuitable because of the development of frazil ice.

As both the fish and the aquatic invertebrates (collectively referred to as bugs) are coldblooded, their metabolic rate decreases with the cold temperature of the stream and the pace of life slows down overwinter. Slows down, but doesn't stop. In the sediment and water column, terrestrial organic material, such as leaf fall,  is processed and used for growth by the assorted invertebrate guilds of shredders, scrapers, grazers, collectors, predators, omnivores, and scavengers.  These bugs then provide food for the stream's fishes.

Although their metabolic rate is reduced by the colder temperature, adult and juvenile fish still need to eat: just ask any ice fisherman. While adult brook and brown trout spend the winter recovering from energy spent in their fall reproduction, spring spawning fish, such as suckers and pike, are busy packing energy into their eggs and sperm, which they will release in April and May. For juvenile fish, winter diet is important to maintaining energy reserves so they can make it through to the next spring.

Mill Creek is also home to a range of mammals Further downstream, beavers are quietly spending the winter in their warm cosy lodges, venturing out now and then to collect the branches they gathered last fall and stored in underwater food banks. They may share their pond and even their lodge with muskrats which feed on under ice vegetation. Meanwhile, mink will forage for fish, mussels, crayfish and even frogs and turtles buried in the mud. Walking along the creek, we see the tracks of coyote, racoon and even wild turkey, and there are deer and rabbit signs in the vegetation to either side of the stream. Yet winter can also be harsh: a frozen opossum lies under one of the cedars, the same stand where we found a deer skeleton a couple springs ago.

But now  the days grow longer. All too soon, Mill Creek's winter tale will be past and a busy spring upon us.

For more information on Mill Creek and its Friends visit http://www.friendsofmillcreek.org/