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Fly fishing report · West
Big Lost River
A Big Lost River report for the Mackay tailwater and valley access, RiverReports/USGS flow checks, IDFG rules, whitefish protection, hatches, flies, and dewatering cautions.
Check flow & weatherToday's fishing decisionMost live data available
39
Skip today
Do not commit until the safety issue clears.
Red Flag Warning issued August 2 at 1:04AM MDT until August 2 at 9:00PM MDT by NWS Pocatello ID
Best supported approachNo approach recommended
Watch nextRed Flag Warning issued August 2 at 1:04AM MDT until August 2 at 9:00PM MDT by NWS Pocatello ID
Updated Aug 3, 1:24 AM UTCUsually refreshes about every 45 minutes
Red Flag Warning issued August 2 at 1:04AM MDT until August 2 at 9:00PM MDT by NWS Pocatello ID
Safety overrides tacticsNo approach recommended
The active safety or closure reason takes priority over wading, bank fishing, and floating guidance.
Check before you drive
Current flow and weather.
Look at whether the river is rising or falling, then check wind, rain, and storms. These can change both safety and fishing.
Loading current flow and weather.
Before you rely on this report
How current is this report?
Conditions can change quickly. Check these dates before you decide to fish, and open the official source when safety or rules matter.
- River conditions
- Checked Aug 2, 2026, 9:24 PM
- Fishing guidance
- Reviewed May 31, 2026
- Public access
- No reviewed map pin available
- Rules and closures
- Official rule links checked
High confidence in the current score inputs.
7 sources are listed below for you to open.
4 public access records are listed. Private spots are never shown.
Source checked Jul 30, 2026
Plan your access
Know where you can enter.
These are public parking, launch, trail, or recreation areas—not secret fishing spots. We never show an angler's private pins, notes, or photos here.
- 0
- Shown on map
- 4
- No map pin
Location checkedDescription only
01No map pin
Area description
Below Mackay Reservoir
The main tailwater and flow-reference reach for this report.
Restrictions and seasonal notes
Irrigation and dewatering can change fishability by reach.
Private land is common enough that maps matter.
02No map pin
Area description
IDFG Big Lost lower access
Official access-site context for lower-river planning.
Restrictions and seasonal notes
Irrigation and dewatering can change fishability by reach.
Private land is common enough that maps matter.
03No map pin
Trailhead
BLM Big Lost River Access Trail
Public-land access context with remote travel considerations.
Restrictions and seasonal notes
Irrigation and dewatering can change fishability by reach.
Private land is common enough that maps matter.
04No map pin
Roadside access
Upper North Fork Big Lost area
A colder and more remote upper-drainage plan with forest-road logistics.
Restrictions and seasonal notes
Irrigation and dewatering can change fishability by reach.
Private land is common enough that maps matter.
How to fish it
Start with the Mackay gauge and reach choice.
The Big Lost is a high-desert trout river where Mackay Reservoir releases, irrigation, and access define the day. Use the below-Mackay gauge, then decide whether you are fishing tailwater, valley, or upper drainage water.
- Use the RiverReports and USGS below-Mackay gauge for the main flow check.
- Check IDFG rules, including mountain whitefish special language.
- Watch for intermittent dewatering and irrigation effects in lower reaches.
- Plan for remote access and limited services away from Mackay.
Why the score looks this way
Public alertLowers score
An active alert is in effect: Red Flag Warning issued August 2 at 1:04AM MDT until August 2 at 9:00PM MDT by NWS Pocatello ID. Check public safety sources before going.
FlowHelps score
USGS shows 399 cfs with a stable over about 6 hours trend. Today's flow is within the normal seasonal range of 348 cfs to 637 cfs. Weather, water temperature, and access are the next checks.
SeasonHelps score
Summer: Caddis, PMDs, stones, and terrestrials matter, but heat and irrigation flows decide the day.
WeatherHelps score
The NWS forecast is about 83F with Patchy Smoke.
Fishing usefulnessHelps score
Skip the Big Lost when flows are too low for responsible trout handling, when IDFG rules or whitefish language are unclear, when private-bank access is the only realistic entry, or when remote weather and road conditions make the drive a poor trade.
Read the water
What changes the plan.
The Big Lost is best when flows are stable enough for safe wading and cold enough for careful trout handling. If irrigation, heat, or dewatering make the river poor, move higher or choose a nearby alternative.
Low clear flow
Use stealth, smaller flies, and avoid pushing fish in shallow warm water.
Stable medium release
Nymph rigs, dry-droppers, and attractor dries can cover riffles and buckets.
High release
Avoid risky crossings and fish softer banks only where access is legal.
Dewatered reach
Do not force a trout plan; move to water with enough cold flow or use a backup river.
Your trip plan
Know when to go—and when to skip it.
Best flows
Use the RiverReports Mackay chart and USGS 13127000 together. Stable tailwater releases create the best window; dewatering, sudden irrigation changes, or very low warm water should move you to a different reach or another river.
When to skip
Skip the Big Lost when flows are too low for responsible trout handling, when IDFG rules or whitefish language are unclear, when private-bank access is the only realistic entry, or when remote weather and road conditions make the drive a poor trade.
Local plan
Start with the Mackay tailwater and IDFG lower access context, then compare BLM trail or upper Forest Service water only if road, weather, and flow conditions support the extra travel.
Backup water
If the Big Lost is dewatered, too warm, or access-limited, compare the Big Wood River, Boise River, or another Idaho trout option after checking current rules and flows.
Hatches & flies
Bring a flexible box.
TimingWhat to watchUseful flies
SpringBWOs, midges, caddisBWO emerger, zebra midge, caddis pupa, pheasant tail
Fly family · report suggests “BWO emerger”Blue-Winged Olive PatternsBWO describes a hatch group, not one fly. Nymph, emerger, dry, cripple, and spinner profiles must stay separate because they occupy different parts of the water column.See family guide ↗
Fly pattern · report suggests “zebra midge”Zebra MidgeLook for a very slim tapered thread body, evenly spaced contrasting wire rib, a small bead, and no tail or wing. The reviewed classic is black with silver wire and a silver bead. Red, olive, brown, glass-bead, jig-hook, resin-coated, or tailed forms must remain labeled variations rather than replacing the classic identity.See photos & how to fish it ↗+ 2 more reviewed guides in the Fly Box Early summerPMDs, caddis, golden stonesPMD dry, elk hair caddis, stonefly nymph, chubby
Fly family · report suggests “PMD dry”Pale Morning Dun PatternsPMD names an insect group, not one fly. Pale nymphs, trailing-shuck emergers, upright or low-riding duns, cripples, and spent-wing spinners stay visibly separate.See family guide ↗
Fly pattern · report suggests “elk hair caddis”Elk Hair CaddisLook for a tented elk- or deer-hair wing, clipped hair head, dubbed body, rib, and hackle palmered along the body. The body color should be labeled because tiers often match different natural caddis colors.See photos & how to fish it ↗+ 2 more reviewed guides in the Fly Box Late summerTerrestrials, caddis, small mayfliesAnt, beetle, hopper, caddis dry, perdigon
Fly family · report suggests “Ant”Ant PatternsAnt patterns can be foam, fur-bodied, winged, or sunken. The narrow waist and paired body lobes matter more than one material recipe.See family guide ↗
Fly family · report suggests “beetle”Beetle PatternsBeetle flies range from simple foam shells to hair-bodied and sunken forms. A rounded back and compact profile distinguish the family from ants and hoppers.See family guide ↗+ 3 more reviewed guides in the Fly Box FallBWOs, midges, October caddisBWO dry, RS2, zebra midge, October caddis
Fly family · report suggests “BWO dry”Blue-Winged Olive PatternsBWO describes a hatch group, not one fly. Nymph, emerger, dry, cripple, and spinner profiles must stay separate because they occupy different parts of the water column.See family guide ↗
Fly pattern · report suggests “RS2”RS2Start with the beadless architecture: two dark-dun Microfibett tails separated behind a slim, tightly twisted and visibly segmented dubbed abdomen; a fuller thorax; and saddle-hackle web clipped into a short angled wing bud. Rim Chung's original-style form uses natural beaver dubbing and hackle web. CDC- or Antron-wing ties, beads, curved hooks, flash, and tailless Avatar-style flies must remain labeled variations.See photos & how to fish it ↗+ 2 more reviewed guides in the Fly Box 01
Check whether the river has enough cold water for the reach you plan to fish.
02
Use the Mackay gauge for tailwater planning, not lower-basin assumptions.
03
Fish banks and riffle edges with dry-droppers during summer.
04
Respect private land and use official access sites when possible.
05
Handle mountain whitefish according to current IDFG rules.
Rules & closures
Check the official rules before you go.
Open the agency page to confirm the exact river section, species, season, tackle rules, limits, and emergency closures.
Official links checkedRules for this river or sectionNo special rule found
We did not find a special rule for this river.
We checked the state source but did not find a clearly supported exception for this river. Check the official rules for the exact section you plan to fish.
Rules checkedJul 30, 2026
Report a correction ↗Always follow the official rule
This summary helps with trip planning. The linked agency rule is the one you must follow.
Updates from anglers
What anglers are seeing.
Recent observations from general areas of Big Lost River. We review every update before it appears and never publish exact fishing spots, private access details, or photo locations.
Angler updates are not loading right now.
You can still use the river plan above. Check back later for recent observations from other anglers.
Where this report comes from
Open the sources we checked.
Fishing guidance checked: May 31, 2026
Site check: Jul 14, 2026. We regularly test report links, live scores, flow and weather data, and page loading. The content date changes only when the fishing guidance or its sources change.
flowRiverReports: Big Lost River below Mackay ReservoirflowUSGS monitoring location: Big Lost River below Mackay ReservoirregulationIDFG Big Lost River Fishing PlanneraccessIDFG Big Lost River lower accessaccessBLM Big Lost River Access TrailaccessUSFS North Fork Big Lost River AreasafetyNational Weather Service forecast point near Mackay
Common questions
Before you leave.
What reach does this Big Lost report cover?+
It focuses on the below-Mackay Reservoir tailwater and valley access, with upper drainage context.
Which gauge should I use?+
Use USGS 13127000 below Mackay Reservoir, shown through RiverReports and official USGS.
Why mention dewatering?+
Irrigation and basin conditions can leave some lower reaches with poor or intermittent surface flow.
Are whitefish rules important?+
Yes. IDFG special rules can apply, so check current Idaho rules before harvest or handling assumptions.