Fort Peck Lake goes on for arm after arm of flooded coulee, and the water you can see from the ramp you launched at is a small fraction of the lake. Flathead is a different kind of day out. It is open water that builds a chop by mid afternoon, and the crossing you made at seven in the morning is not the crossing you get coming home. Canyon Ferry does the same thing on a smaller sheet, close enough to town that people get caught out on it. Lake Koocanusa is long and narrow and sits at whatever level the dam is holding that month, so the shoreline you tied up against last year may be a hundred yards of dry gravel this year. None of this is coastal boating, and there is no NOAA chart for a reservoir.
The Montana pack carries 8,645 charted water bodies. Of those, 2,096 carry a name, and 176 of the named ones are larger than a square kilometer. Among the 48 state lake packs, this one sits tenth for total water bodies, tenth for named water, and eleventh for lakes over a square kilometer, which puts it in the top quarter on all three counts. The water bodies come from the National Hydrography Dataset. The depth under them comes from state elevation surveys.
Tenth of 48 is a useful thing to know about a pack, because it tells you the density is there before you go looking for a specific lake. The named water is where most people start: 2,096 water bodies with a name attached, also tenth among the packs. The other 6,549 are charted water carrying no name at all, which is where the small stuff lives. The pothole a mile off the county road is in there. So is the stock reservoir at the end of a two-track.
A square kilometer works out to about 247 acres, so the 176 named lakes above that threshold reach a long way down the list. Benton Lake at 4,300 acres is only the twentieth largest the pack carries, which means another 156 named lakes over a square kilometer sit below it. The water you actually launch a trailer on in this part of the country is mostly in that lower stretch of the list rather than in the handful of names everybody outside the state has heard of.
| Lake | Charted surface area |
|---|---|
| Fort Peck Lake | 938 km² (231,735 acres) |
| Flathead Lake | 491 km² (121,378 acres) |
| Yellowstone Lake | 340 km² (83,966 acres) |
| Canyon Ferry Lake | 132 km² (32,544 acres) |
| Lake Koocanusa | 116 km² (28,639 acres) |
| Hungry Horse Reservoir | 95 km² (23,426 acres) |
| Lake Elwell | 52 km² (12,973 acres) |
| Bighorn Lake | 50 km² (12,331 acres) |
| Hebgen Lake | 50 km² (12,331 acres) |
| Medicine Lake | 36 km² (8,945 acres) |
| Noxon Reservoir | 32 km² (7,809 acres) |
| Island Park Reservoir | 30 km² (7,512 acres) |
The twelve largest named lakes the Montana pack carries, by charted surface area, measured from the chart polygon itself. The pack carries 8,645 charted water bodies in all, 2,096 of them named, and 176 named lakes larger than one square kilometer.
A federal agency surveys the coast from a ship. Nobody does that to a reservoir behind a dam on the Missouri. Inland depth in Helm is built from state elevation surveys instead, and that single substitution is the difference between a lake drawn as a blue shape with a label on it and a lake you can draw a line across and get a depth profile back from, read against the draft you entered once.
A reservoir also sits at whatever level it is being held at, and that is the thing to keep in your head while you read a contour on one. The contour is the shape of the bottom. The bottom does not move. What moves is how much water is standing on it, and on Hungry Horse Reservoir or Lake Koocanusa late in the season that difference is large. The old creek channel down the middle is the same line in October that it was in June. What changed is how much of the flat on either side of the channel is still wet, and how far out from the bank you have to be before the shoulder drops away.
Read the contour as the bottom and the current level as the water on top of it, and the two together tell you what is under the boat at the hour you are out there. That is also why the shading around a cove mouth shifts from one trip to the next on water that gets pulled down, while the channel stays open the whole time.
Fort Peck Lake is the largest piece of water in the pack at 937.8 square kilometers, or 231,735 acres. To put that in proportion, it is bigger than Flathead Lake and Yellowstone Lake added together. Planning a day on it is less about the distance across than about which arm you are committing to, because the arms run out of sight of one another and the wind in one is not the wind in the next.
Flathead Lake at 491.2 square kilometers, or 121,378 acres, is close to four times the surface of Canyon Ferry Lake. Canyon Ferry at 131.7 and Lake Koocanusa at 115.9 are near enough in size to be in the same class, though they fish nothing like each other. Further down, the numbers start repeating: Bighorn Lake and Hebgen Lake both measure 49.9 square kilometers, 12,331 acres, taken off the chart polygon itself.
These are the twenty largest named lakes the pack carries, with the surface area measured from the chart polygon.
The packs are cut to a box rather than to a state line, so water near a boundary lands in more than one of them and is charted the same way in each. Lake Koocanusa runs north toward the Canadian line. Bighorn Lake runs south down the canyon. Henrys Lake and Island Park Reservoir sit at the other corner of the box entirely. If the water you fish is close to a line, the neighboring pack covers it from its own side, and the Idaho, Wyoming, South Dakota and North Dakota packs are the ones that share an edge with this one.
Pull the pack onto the phone at home and the charts, the depth, the routing and the measure tool keep working with no bars, which is the normal condition at a reservoir ramp two hours from anything. Fort Peck is the obvious case. So is most of the Hi-Line, and so is any forest road running down to a lake in the northwest corner.
Inland lake depth comes from state elevation surveys rather than from nautical charts, and a reservoir sits at whatever level it is being held at that week. Treat the shape and the depth as a guide, keep a lookout, and verify locally.
Fort Peck, by a wide margin. Measured off the chart polygon in the pack, Fort Peck Lake is 937.8 square kilometers, or 231,735 acres, and Flathead Lake is 491.2 square kilometers, or 121,378 acres. Fort Peck is larger than Flathead and Yellowstone Lake put together, and those three are the largest pieces of water the pack carries.
The pack carries 8,645 charted water bodies. Of those, 2,096 carry a name, and 176 of the named ones are larger than a square kilometer, which is about 247 acres. That is tenth of the 48 state packs for total water bodies and for named water, and eleventh for lakes over a square kilometer.
Yes, as long as you read them for what they are. A contour is the shape of the bottom, and the bottom does not move. The water level does, so on Hungry Horse Reservoir or Lake Koocanusa in the fall you read the contour together with the level the dam is holding. Inland depth in Helm comes from state elevation surveys rather than from a nautical chart, which is why a reservoir has contours on it at all.
Helm Boating