Mill Creek is a groundwater-fed stream located in the Township of Puslinch, in southwestern Ontario. Mill Creek is part of the Grand River watershed.

The Friends of Mill Creek (FoMC) community group organizes ecological restoration activities on Mill Creek, including a youth stewardship rangers program.

Geographical location Mill Creek is located in the centre of the Grand River Watershed in south-central Ontario. Extending into the City of Cambridge and Township of North Dumfries, the City of Guelph, the Township of Puslinch, and the Town of Milton, the subwatershed drains around 104 square kilometres of land. The headwaters begin in Puslinch Township and travel southwest through Aberfoyle, then through Class I wetlands just south of Highway 401. After reaching the Shade's Mills Reservoir and Dam, Mill Creek outlets into the Grand River in downtown Cambridge near the Grand River Pedestrian Bridge.

A map of the Mill Creek subwatershed is available on the Friends of Mill Creek website.

History The Grand River Conservation Authority (GRCA) declared Mill Creek as, "one of the most significant examples of stream improvement work in the province" in 1991.Warren H. Yerex. (1991). Mill Creek - A Resilient Resource. Page 1 of the 1991 Mill Creek Spawning Survey.

The integrity and ecological status of Mill Creek has historically faced many threats. Historical threats to the creek include sediment loading and channelization due to the development of Highway 401 and the Hanlon Expressway interchange; as well as unrestricted cattle access to the creek which exacerbated bank destruction.  Currently, further development in the area may damage the creek.

Efforts for stream restoration at Mill Creek have historically been motivated by the want to maintain trout populations. Mill Creek was originally a brook trout stream, and brown trout were introduced in 1940 and 1950. Between 1982 and 1991, the human resources for creek rehabilitation were provided by a variety of Special Employment programs: the KW Fly Fishers, Galt Sportsman Club, Anti-Recession programs, and summer students.  In 1993, the Ontario Ministries of Environment and Energy and of Natural Resources published subwatershed planning guidelines.Ministry of the Environment - Standards Development Branch, Ministry of Natural Resources - Land Use Planning Branch, Queen's Printer for Ontario (1993). Subwatershed Planning.   The goal of these guidelines was to promote an early, integrated, effective, and ecosystem-based approach to land and subwatershed planning.  This document served as a basis for the Mill Creek Subwatershed Plan. The subwatershed plan was published in June 1996, which characterized the subwatershed environmental system and proposed a management plan for the creek and surrounding area.Grand River Conservation Authority (GRCA). (1996). Mill Creek Subwatershed Plan. Prepared by CH2M Gore & Storrie Limited, Gartner Lee Ltd., RBA Planning Consultants Ltd., Salter Research Services, Schroeter & Associates Ltd., & A. Wayne Caston Consulting Services Ltd. https://www.cambridge.ca/en/your-city/resources/Mill-Creek-Subwatershed-Plan.pdf  

Biodiversity

Valued ecosystem components Valued ecosystem components (VEC) are borne from a combination of scientific and societal ideals, and provide insight into impact analysis, mitigation, and monitoring.Stillwater Canada Inc. (2012). Section 2.0: Project Scoping. https://ceaa-acee.gc.ca/050/documents_staticpost/54755/57673/2_-_Project_Scoping.pdf   In Mill Creek, two species that unmistakably serve as VECs are brook trout and brown trout. Both species inhabit some but not all stretches of Mill Creek, and are highly sought after by the local angler community.Dhiyebi, H., Courtenay, S., & Servos, M. (2018). Cumulative Effects in the Mill Creek Subwatershed.

The cool and well-oxygenated water of Mill Creek is the ideal habitat for brook trout that optimally live at 14 °C to 16 °C. Some populations of brook trout are amphidromous and others seasonally migrate between lakes and tributary streams. When not migrating between streams, brook trout can also move great distances within streams in response to seasonal and spawning needs – which occur in the late summer and fall. They are opportunistic feeders, with a diet comprising both vertebrates and invertebrates. Brook trout are an ecologically important species, having the capacity to affect the growth and survival of cohabitating species through competition, preying, or hybridization.

While brook trout are native to Eastern North America, brown trout are native to Europe, North Africa, and West Asia and have been in and naturalized to Ontario streams in 1913. The provincial government has been stocking brown trout in certain stretches of Mill Creek on an annual basis since at least 2014. The number of individuals stocked in the creek nearly doubled from 507 in 2023 to 1000 in 2024.

Brook trout can survive in a large range of temperatures and oxygen concentrations, being found throughout stream-fed pools and ponds, calm, quiet waters, and faster, turbulent streams. They are capable of inhabiting 0 °C to 27 °C but can only reproduce between 0 °C to 10 °C; spawning occurs in October and November. Some brown trout display amphidromous behavior, while others remain for their entire life in their home stream. These fish feed most aggressively during the night and are opportunistic feeders. What makes brown trout ecologically important is their capacity to survive in a broader temperature and oxygen range than other native species, namely the brook trout. In this respect they have the capacity to prey on and out-compete brook trout, altering the naturally occurring fauna of Ontario streams.

Invasive species

Within the Grand River watershed, there are several invasive species of concern that may be currently present in Mill Creek or invade the region in the future.

Aquatic invasive species of concern include Neogobius melanostomus (round goby), Petromyzon marinus (sea lamprey), Faxonius rusticus (rusty crayfish), and Dreissena polymorpha (zebra mussels). Round gobies are invading inland water from the Great Lakes, where they negatively impact ecosystems by eating the eggs of native fish. The GRCA reports that round goby is present as far north in the watershed as Belwood Lake, and the species has been detected in most of the systems surrounding Mill Creek. Parasitic sea lamprey have been detected as far north as the Caledonia Dam in the Grand River watershed, so are not present near Mill Creek yet - although they are likely moving north. Rusty crayfish are spreading through southern Ontario (including the Grand River watershed), and have been detected by the Ministry of Natural Resources near Mill Creek. These crayfish outcompete native crayfish and juvenile fish. Zebra mussels have been detected within the Grand River watershed up to the Dunnville Dam, as well as in Guelph Lake - in the vicinity of Mill Creek.

Invasive plants present in the Mill Creek subwatershed include Rhamnus cathartica (European/common buckthorn), Alliaria petiolata (garlic mustard), Phragmites australis (common reed grass), and Lythrum salicaria (purple loosestrife), which threaten native plant species as well as the integrity and functioning of wetland and riparian habitats.

Invasive pathogens that may affect species in the Mill Creek ecosystem include viral hemorrhagic septicemia (VHS), ophidiomycosis (snake fungal disease), Ranavirus spp., and chytridiomycosis. The pathogen VHS (affecting fish) is present in Lake Erie and parts of its tributaries; Mill Creek is located next to a VHS management zone. The fungal pathogen Ophidiomyces ophiodiicola affects native snake species that are present in the Mill Creek subwatershed, including Thamnophis sirtalis sirtalis (eastern garter snake), and has been detected in this region. Viruses in the genus Ranavirus infect reptiles and amphibians, and are distributed across most of Canada. The strain Frog virus 3 (FV3) is prevalent in eastern North America and threatens the frogs, salamanders, and turtles present in the Mill Creek subwatershed. The fungal species Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans cause chytridiomycosis in amphibian species, leading to detrimental levels of mortality. These pathogens are widespread in wild amphibian populations in Ontario, and have been detected in the vicinity of Mill Creek. Invasive pathogens affecting trees in this region include Dutch elm disease and white pine blister rust - among others.

Species at risk The Jefferson salamander (Ambystoma jeffersonianum) was assessed as threatened in 2008 and later changed to endangered in 2011. The two chorus frog (Pseudacris triseriata) species in Ontario are the western and the boreal chorus frog, which are both more often heard than seen. Monarchs (Danaus plexippus) only breed in habitats with milkweed (Asclepias spp.), since it is the only plants the caterpillars feed on.NCC: Nature Conservancy Canada. (2024). Monarch. https://www.natureconservancy.ca/en/what-we-do/resource-centre/featured-species/insects-and-spiders/monarch.html   There are many birds at risk throughout the Mill Creek Subwatershed. Some common habitats that need to be protected for these birds to thrive include a mix of tree species, including deciduous, coniferous, and mixed forests, a mix of forested areas and meadows for foraging and nesting,Government of Ontario. (n.d.b) Eastern whip-poor-will. https://www.ontario.ca/page/eastern-whip-poor-will#:~:text=Eastern%20Whip%2Dpoor%2Dwill%20is%20usually%20found%20in%20areas%20with,resting%20and%20sleeping)%20and%20nesting.     rocky areas, diverse areas including some with little to no vegetation,Government of Ontario. (n.d.c) Common nighthawk. https://www.ontario.ca/page/common-nighthawk and diverse ages of plants.Government of Ontario. (n.d.d) Eastern wood-pewee. https://www.ontario.ca/page/eastern-wood-pewee     Activity that threatens the survival of these species includes outdoor cats attacking nests on forest floors and in lower areas, habitat loss and degradation, car mortalities and changes in food supply related to pesticides, and habitat loss related to fire suppression. Some threats to the survival of turtle species in Mill Creek include loss or fragmenting of habitat, motor vehicles, raccoons and foxes that prey on eggs, as well as illegal collection for the pet trade,Government of Ontario. Blanding's turtle. https://www.ontario.ca/page/blandings-turtle   and adult mortality since many turtles are slow breeders.Government of Ontario. Snapping turtle. https://www.ontario.ca/page/snapping-turtle   Habitats in Mill Creek where the eastern milksnake (Lampropeltis triangulum) would be found include meadows, deciduous, coniferous, and mixed forests, areas of high forest cover, man-made structures such as barns for hibernation, near water sources such as marshes and swamps. The dense blazing star is a perennial wildflower that can grow as tall as two meters. It grows in moist prairies and wet areas. It does not do well in the shade and is usually found in sunny areas kept open by fire, floods, drought, or grazing.Government of Ontario. Dense blazing star. https://www.ontario.ca/page/dense-blazing-star   It is threatened by habitat loss due to development or habitat alteration, as well as invasive plants such as European common reed (Phragmites australis).

Physical attributes Aggregate pits CBM McNally Pit