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John A. Weeks III
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Highways, Byways, And Bridge Photography
Thomson Dam Gate House
Saint Louis River Water Project
Thomson, MN

Thomson Dam Gate House

• Location: River Mile 29
• River Elevation (Entrance): 1,069 Feet
• River Elevation (Exit): 1,051 Feet
• Date Built: 1907
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 gate house.

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.


Thomson Dam Gate House
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 view.

Thomson Dam Gate House
Thomson Dam Gate House
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.

Thomson Dam Gate House
Thomson Dam Gate House
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.

Thomson Dam Gate House

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