Gibson Dam fills it, the Diversion Dam empties it
The water below Augusta is managed water, run through the Sun River Project before you ever see it. The Bureau of Reclamation built Gibson Dam between 1926 and 1929 to store the Rocky Mountain Front's snowmelt for roughly 93,000 irrigated acres in the Sun River Valley. About 3 miles below the dam the Sun River Diversion Dam pulls water into the Pishkun Supply Canal and sends it to Pishkun Reservoir, which the Greenfields Irrigation District runs, and Willow Creek Reservoir belongs to the same system. The river an angler fishes is what is left over after the canals take their share.
FWP puts it bluntly on its Sun River page: the river below Gibson Dam has historically had dewatering problems tied to agricultural water use. In spring 2019 its crews could sample only one site because suitable flows lasted so briefly, and they estimated 308 trout of 8 inches and longer at a monitoring site near Augusta, a figure the agency called well below the North Fork Sun, the South Fork Sun and the Smith. FWP holds instream flow reservations of 100 cfs from the Diversion Dam to Elk Creek, which comes in at Gilman below Augusta, and 130 cfs from Elk Creek to the mouth at Great Falls. Those numbers are a floor on paper, not a promise in August.
Fort Shaw, Medicine River and Largent's Bend
Montana lists the Sun as a Class I stream from Gibson Dam to the Missouri, so the bed and banks to the high-water mark are open once you are legally in the river. Getting in is the hard part, because almost all of the valley is private farm and ranch ground and FWP has only three fishing access sites on the mainstem, all of them in the lower half.
Fort Shaw FAS is half a mile north of Highway 200 in the town of Fort Shaw on North Fort Shaw Road, a 3-acre site at the bridge with a toilet and a hand launch. Medicine River FAS, which carries the Sun's other name, is half a mile south of Highway 200 at the town of Sun River on the Sun River Cascade Road: 10.6 acres, a vault toilet, a hand launch, and archery-only hunting in fall. Largent's Bend FAS, 5 miles west of Vaughn on Highway 200, is the big one at 168.9 acres, but it has no boat facilities and a large pond takes up part of it. Between Simms and Augusta there is no FWP site at all, so plan on bridge rights-of-way or a landowner's permission.
What FWP's shocking boats find at Simms and the 287 bridge
The Sun is a brown trout river with rainbows and a lot of mountain whitefish. FWP electrofishes two long-running sections, one at Simms and one at the Highway 287 bridge near Augusta, and the fish it measures there are larger than the low population estimate might suggest. In the 2024 Highway 287 runs the brown trout averaged 400 to 428 millimeters depending on the pass, roughly 16 to 17 inches, and at Simms that year the biggest brown measured 599 millimeters, better than 23 inches. The Simms passes in 2025 averaged 361 to 375 millimeters for browns. Rainbows show up in every sample in smaller numbers, and whitefish often outnumber everything else: one 2023 Simms pass handled 176 of them.
Fewer fish than its neighbors, then, but fish that grow. Streamers along cutbanks and big nymphs in the deeper runs make sense, and so does covering ground: FWP counted 9,847 angler days on the reach from river mile 17 up to 96 in 2020, spread over a lot of water.
The 2026 rulebook, the hoot-owl stretch and the bridge closure
The Sun's mainstem has no special listing in the 2026 FWP regulations, so Central District standard rules apply below Gibson Dam: rivers and streams open all year; combined trout 3 daily and in possession, only 1 over 18 inches and only 1 cutthroat; arctic grayling released immediately; mountain whitefish 20 daily and 40 in possession. Above the dam the rules change. Gibson Reservoir, from the dam up to where the North and South forks meet, allows 5 trout with grayling catch-and-release, and the North Fork and South Fork and their tributaries allow 3 trout with only 1 over 12 inches.
Summer brings emergency rules. On July 11, 2026, FWP put hoot-owl restrictions, no fishing from 2 p.m. to midnight, on the Sun from the Highway 287 bridge to the mouth of Muddy Creek, and lifted them effective August 29 when water temperatures moderated. The agency's drought policy triggers those limits when daily maximum water temperature hits 73 degrees for three days running. Separately, the river is closed to all use, boating and wade fishing included, from 500 feet above to 500 feet below the Sun River Bridge, also called the Pishkun Road Bridge, from June 1 through November 15, 2026, while the Federal Highway Administration replaces it.
Why Simms, and what 130 to 309 means
Use USGS 06085800, Sun River at Simms. It sits at about river mile 40, below the Elk Creek confluence and above Fort Shaw, in the middle of the trout water, and FWP lists it on its Sun River page along with the Vaughn gauge at mile 14. Vaughn reads the warmer lower river, and the gauge below Willow Creek near Augusta reads the water just below the canals.
No shop publishes a wading range for the Sun, so the 130 to 309 cfs range on this page comes from two public numbers. The low end is FWP's 130 cfs instream reservation for the Elk Creek to mouth reach. The high end is the July mean at Simms in the USGS record from 1966 to 2026. Monthly means there run 230 cfs in March, 304 in April, then 1,204 in May and 1,822 in June; after that they drop to 309 in July, 149 in August and 146 in September before rising to 203 in October as irrigation shuts off. Below the range, fish are crowded and warm. Above it before July, the Sun is usually carrying runoff and is poor for wading.
Bugs from a western Montana chart, applied east of the Divide
No shop we could find publishes a Sun River hatch chart, so the table here follows Grizzly Hackle's western Montana chart and should be read as a regional guide, not a record of this river. Expect midges in winter, blue-winged olives in March and April and again in September and October, caddis from spring into late summer, pale morning duns and golden stones in early summer, then tricos and hoppers in August and September, the same weeks the 2026 hoot-owl rule was in force. Streamers are worth carrying all year on a river whose best fish are browns.