The Thomson Water Project is a large and complex hydropower generation plant
located on the Saint Louis River. Water flows into the Thomson Reservoir,
which backs up behind the Thomson Dam. Some of the water flows down the
historic Saint Louis River channel, while water flowing to the power plant
flows 2 miles down Forbay Canal, and then a mile down a set of large penstock
pipes, where it flows through the turbines and back into the river.
The Thomson Dam Gate House is located on the southeast corner of the Thomson
Reservoir. It regulates the flow of water from the reservoir and into Forbay
Canal. The gate house has 8 water outlets, which appear to be in two
buildings set side by side. It appears that there originally was a gate house
with 4 gates, and an additional 4 gates were added at a later date. The
gate house building are approximately 84 feet wide (combined), and about 20
feet from front to back. There is a 205 foot concrete dam running southwest
from the gate house, and a 150 foot concrete dam running northeast from the
The Gate House is visible from the Thomson Road Bridge by looking northwest up
the center of the Forbay Canal. The photo above is a telephoto view of the
gate house from that vantage point. The dividing line between the two
building halves is readily visible in this photo, and while the building on
the right appears to be the older building, I am not able to tell for sure
either way. There is a trail along the south side of the Forbay Canal that
leads back to the Gate House area. It is about a 20 minute walk that has a
few challenging spots. While the trail is well worn, this appears to be
private property that might be watched by video cameras.
These two photos are views looking upstream to the northwest from the deck
of the Thomson Road Bridge. The gate house is 1,550 from this vantage point.
The photo above is a late fall view, while the photo below is a mid-summer
These two photos are views of the Gate House building. The photo above is
the southwest side of the building as seen from the top of the dam structure.
The photo below is a view of the northwest side of the building, which faces
the Thomson Reservoir. This view is possible because of a peninsula that
extends past the dam out into the reservoir. This peninsula is used to store
driftwood and debris that backups behind the Gate House. This view also
shows that the Gate House is two distinct buildings built at different times.
I am not sure which is the original structure and which is the addition.
The photo above is another short section of concrete dam built to fill in a
gap between rocks along the south side of the Thomson Reservoir. This dam
segment is located just west of the gatehouse. The photo below is a rock
pile located along the south side of the Forbay Canal. The section of canal
just downstream of the Gate House was blasted from rock known as slate and
greywacke. These were mud and sand beds from an ancient sea about 1.9-billion
years ago. The rock was later uplifted causing it to fold and fracture,
resulting in long rock ridges that are exposed at the surface through the
Thomson Reservoir and Jay Cooke State Park area.