For Sale

Cromwell Island

$72,000,000 Dayton, MT 348± Deeded Acres

Executive Summary

Like the cold, clear waters of Flathead Lake, mystique surrounds Cromwell Island. It overlooks the legendary Wild Horse Island, where Salish and Kootenai historically swam their horses to keep them safe from rival tribes. In the 1930s, construction started on a dam near Polson, Montana, at the outlet of the Flathead River. Prior to its completion, in periods of low water, Cromwell Island became a peninsula connected to the shore. It is all but certain that the Native Americans historically herded their horses onto this peninsula before swimming them across to Wild Horse. Today, Cromwell Island is a full-time island, the entirety of which is now for sale.

Flathead Lake, with almost 200 square miles of surface area and 185 miles of shoreline, is the largest natural freshwater lake west of the Mississippi River — Yes, even bigger than Lake Tahoe! To our knowledge, Cromwell Island, with almost 350 acres and nearly three miles of shoreline, is the largest private island in freshwater west of the Mississippi River that is under a single ownership.

Like Loch Ness, Flathead even has its own monster. While the lore of a huge antlered monster living in the lake is rooted in the ancient oral histories of the Kootenai Tribe, recorded sightings of the creature date back to 1889 when a steamboat captain along with 100 passengers spotted a “whale-like” object on the water that dove after being shot at by one of the passengers. Somewhere along the way, locals dubbed the monster “Flessie”, an obvious “hat tip” to her counterpart in Scotland.

Cromwell Island was purchased in the late 1980s by Robert M. Lee, a renowned automobile and antique arms collector, explorer, author, and conservationist. Before his death in 2016, Mr. Lee and his wife, Anne, partially completed construction of a monumental structure including over 45,000 square feet of living space that was to serve as their home full-time. Even in its unfinished state, it has a magnitude of presence reminiscent of Versailles.

The mystique endures as Cromwell Island is now for sale and, with so many possibilities, a new owner will have the opportunity to write the next chapter of a storied place.

General Description

Cromwell Island is “squarish” in shape, with the topographical summit of the island (3,400 feet above sea level) being situated almost dead center of the landmass. The shoreline sits at about 2,900 feet. Forty percent of the island is forested, while 46% is classified as grazing land. About 13% is irrigated, and the remainder is currently assessed as homesite. The views from the island are off the charts. The Mission Mountains hold snow most of the year and create a jagged horizon of granite to the southeast. The waters of Flathead Lake are as clear as gin. The rocky subsurface topography forms a pallet of colors ranging from an almost neon turquoise to the deepest shades of cobalt and navy blue. At times when the sun is right, it appears that boats are floating in the air, much like a scene from the Caribbean Sea. 

A ferry landing (large boat ramp) on the mainland provides year-round access to the island. 

Broker's Comments

An island offering is always unique. An offering to own the largest wholly owned private island west of Mississippi River is near fantasy. Cromwell Island, while planned as a personal residence, could be equally well-suited for development or a private club. 

Learn about the locale


Cromwell Island is located approximately a half mile west of Wild Horse Island near the west shore of Flathead Lake just south of Dayton, Montana. The west shoreline of Cromwell is approximately 650 feet east of the west shoreline of Flathead Lake. Travel time is about one and a half hours to Missoula International Airport, while Glacier Park International Airport near Kalispell is less than one hour away. Physical access to the island can be had via boat, helicopter, or seaplane.  


Cromwell Island is located near the north boundary of the Flathead Indian Reservation in Western Montana. Flathead Lake is an epic drama of a landscape. It marks the southern limit of a structural depression known as the Rocky Mountain Trench, which extends northward through British Columbia to the headwaters of the Yukon. The lake was formed by a glacial dam of the Flathead River, which enters at the lake’s north end, while the outlet is at the southern end next to the town of Polson. Kerr Dam, completed in 1938, raised the lake level by 10 feet though it was already the largest lake west of the Mississippi. The dam provides power and irrigation infrastructure for the area. The rugged Mission Mountains rise dramatically out of the lake’s eastern shore, while the timbered mountains west of the lake roll on the horizon.

There are numerous quaint towns surrounding the lake, including the charming town of Lakeside, Montana, which is only 15 miles north of the property, and Bigfork, which is located in the northeast corner of the lake. 


The climate around Flathead Lake is milder than one might expect from such a high Latitude. July and August will see average temperatures peak in the mid to high 80s with cool nights in the 50s. There is no month of the year when the average highs fall below the freezing mark, and average lows are in the 20s in mid-winter. Snowfall totals average 44 inches annually in nearby Dayton, Montana, and annual rainfall averages 18 inches. 

Learn more about the property

Acreage (Deeded & Leased)

Deeded Acres: 348±
Total Leased Acres:
Total Acres: 348±

Additional Information

A 1961 custom-built barge with a length overall of 60 feet is included in the sale. It has twin Vortec V6 engines with Volvo Penta Outdrives. Pontoons designed for heavy transport have been added as well as a spud system. It was used extensively to ferry eight cubic yard concrete trucks across at a time. A crane with a lift capacity of one ton is also mounted on the barge. An aerator is mounted on the stern so ice formation is mitigated around the engine in wintertime. There are also five agitators (hardwired with 220 volts from shore) that are spaced and submerged across the channel allowing for an ice-free passage throughout winter.


There are two primary structures, a main home and a guest villa, located on the island supported by a mountain of infrastructure. The current owner and her late husband began construction in the 1990s following their acquisition of the island. The effort continued for roughly a decade before construction was halted when the owners decided to relocate their future to Lake Tahoe. 

The Villa

“The Villa”, as it is called, was designed to resemble a 16th-century French architectural masterpiece. Though unfinished, this 45,000+ square foot structure is extraordinary in stature and style. It was originally designed to have 3-4 bedrooms with 9 bathrooms, but the free span nature of the construction would allow for a complete redesign of the interior floor plan to accommodate many uses ranging from a primary residence to a clubhouse for a luxury boutique resort. 

The concrete shell was cast in place and reinforced with epoxy coated rebar. From the ground up, every component of construction was employed with the idea that it should last over a thousand years. The color of the masonry on the walls is reminiscent of the Grand Canyon of the Yellowstone for good reason. This “Montana Travertine” comes from near Gardiner, Montana, just outside the north gate of Yellowstone National Park. It is a “Battered Masonry” installation with 4’ x 2.5’ stones for corners. All cut stone is Dolomitic Limestone with a “tapestry finish” and was sourced from Vetter Stone in Minnesota. It is very hard (Class 2 psi) and handles freeze thaw cycles exceptionally well. 

The terracotta tile roof is from Ludowici, an Italian manufacturer. It is grouted in at the face on a seismic chain. The windows, doors, fascia boards, soffits, and freeze boards are Swietenia Mahogany, a dense rainforest wood that came out of a Caribbean plantation. The windows have tilt-turn hardware and the doors have custom manufactured solid brass olive knuckle hinges. The tile on the decks is Hautville that has been cut in 24-inch squares that are 1.25 inches thick. They are set flat in sand allowing proper drainage without the need for any pitch. 

The twin symmetrical and suspended staircases inside the main entrance are inspired by the hanging staircase in the Nathaniel Russell House in Charleston, South Carolina, which was constructed in 1908. There is also an impressive helical staircase in the north tower.

Three-phase electrical service to the island is provided via a submerged cable. It is sized to handle everything previously contemplated to be built by the current owner, with plenty of headroom if other structures are desired. 

All utilities are housed remotely between The Villa and The Guest Villa so as to minimize any noise interference with the quiet enjoyment of The Villa. There are redundant boilers and pumps built to serve the radiant floor heat that is installed throughout.

With three 8,000 gallon diesel fuel tanks, the 750 kW Caterpillar backup generator is capable of generating off-grid power for 8-12 weeks.  

There are five fire vaults, each with a two-inch line capacity: three at the Villa, one at The Guest Villa, and the other near the dock. There is also 30-40k gallons of water storage buried under The Villa.

The Guest Villa

A short distance away, the Guest Villa is, in essence, finished. Though it is livable today and ready for occupancy, there are a few finishing touches that a new owner can style to their preference. It would be equally suitable for use as a caretaker’s residence or a guest home. All utilities for the Guest Villa are delivered through a distribution network in underground tunnels from The Villa. 

Other Structures

The boat dock has five slips varying in size. Originally built for a 65 foot yacht, the largest slip is believed to be capable of handling a yacht up to 70 feet. There is an underground shooting range located in the area of the main villa that accommodates targets up to a 100 yard distance. The only other structures on the island are temporary. For deeply sentimental reasons, the “hospitality” home currently located along the island’s east shoreline next to the dock area, is scheduled to be moved to the mainland in the spring of 2022, where ownership will be retained by Mrs. Lee. 

Water Rights

The island has two functioning wells. One well, servicing the Villa and Guest Villa, is documented to pump at a rate of 30 gallons per minute from a depth of 290 feet, and the other, servicing the dock area, is documented to pump at a rate of 24 gallons per minute from a depth of 340 feet.

There is also a Permit to Appropriate Water pumped directly from Flathead Lake for 399.66 acres at a flow rate of 1,316 gallons per minute for irrigation, lawn and garden, recreation, and stock.


Property taxes are estimated at $131,075.17 based upon past years.

Learn about the recreational amenities

Wildlife Resources

The ranch is home to a herd of mule deer and is often visited by others that swim across the channel. Wildlife cameras stationed on the island have captured images of bear, fox, and mountain lion. Bird species are plentiful, including raptors, waterfowl, and at least one game species: Hungarian Partridge (commonly known as “Chukar”). 

Cromwell has a front row seat for Wild Horse Island, with its closest shoreline being only 2,500± feet away from Cromwell Island. A spotting scope mounted on a tripod is a great way to view the Wild Horses that still live there as well as some world record Bighorn Sheep.

Recreational Considerations

Pleasure boating and other water sports are also wildly popular on Flathead Lake. One doesn’t normally associate sailing as an activity in the Northern Rockies. However, consistent wind patterns here create highly favorable conditions for sailing that are considered by some to be the finest sailing in North America. As the flat plains west of the lake heat up during the day, they pull cold air from the eastern mountains across the large body of water. This consistent system creates easterlies (winds that blow from the east) that start about mid-morning and blow until around 6 in the evening, setting the stage for active sailing. 

Fishing on Flathead Lake is superb with the most popularly targeted species being Lake Trout. Brought from the Great Lakes, Mackinaw (Lake Trout) were introduced to Flathead Lake in 1905 and have flourished. The average catch is around 8 pounds with many fish exceeding 20 pounds. These large fish are primarily caught by trolling in deep water. However, in the fall the fish move to shallower water and can be caught with a fly rod near the shoreline. The lake also holds pike, perch, whitefish, kokanee salmon, rainbow trout, and some bull trout. 

Whitefish Mountain Resort, less than 60 miles away from Cromwell Island, is easily accessible for a day of downhill skiing. 

Glacier National Park is the crown jewel of the Crown of the Continent, the largest intact ecosystem in the United States. The majestic grandeur of this region is hard to put into words. The west entrance to the park is just over 60 miles from Cromwell Island. 

Whether you like to hike, mountain bike, ride horses, shoot, fish, boat, or just watch wildlife, you will not be disappointed in Western Montana. 

Back to Overview


Cash at closing. The current owner will be removing the “hospitality cabin” next to the dock in the spring of 2022.  

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