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 & weather

Today's river scoreHigh source confidence

Caution

Best option: Wade.

Wading is in play only where your chosen access has clear footing, legal entry, and no forced crossings.

Updated Jul 13, 11:17 PM UTCUsually refreshes about every 45 minutes

Recommended approachWade

Mode scores adjust the river-wide score for the risks of wading, bank fishing, or floating.

Wade · Best fit48/100

Wading is in play only where your chosen access has clear footing, legal entry, and no forced crossings.

Bank / edgeCheck

This report does not describe this as a primary mode. Verify legal access, depth, launches, and retreat options before planning around it.

Float48/100

A float is in play where this report supports boat access and wind, releases, and shuttle logistics are manageable.

Confirm before you leave

Flow and weather right now.

Use the flow trend to confirm the score before you leave. Weather can change the safest and most productive fishing window.

River strategy

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 this score moved

FlowUse caution

USGS shows 438 cfs with a stable over about 6 hours trend. same-date USGS history (1904-2024, 112 readings) puts normal around 609 cfs and the lower quartile near 466 cfs; today's flow is below normal for the date. This is below normal, so edge depth, temperature, and pressure matter.

HeatUse caution

The NWS forecast is near 88F. Fish early and verify water temperature where trout stress is possible.

Public alertUse caution

A heat alert is active near this forecast point, so the score is capped until water temperature and fish-handling risk are checked. NWS alert: Heat Advisory issued July 13 at 2:50AM MDT until July 13 at 9:00PM MDT by NWS Pocatello ID.

Best mode nowUse caution

Wade: Wading is in play only where your chosen access has clear footing, legal entry, and no forced crossings.

SeasonHelps score

Summer: Caddis, PMDs, stones, and terrestrials matter, but heat and irrigation flows decide the day.

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.

Field plan

Fish it with intention.

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

Reviewed family · report says “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

Reviewed pattern · report says “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

Reviewed family · report says “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

Reviewed pattern · report says “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

Reviewed family · report says “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

Reviewed family · report says “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

Reviewed family · report says “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

Reviewed pattern · report says “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

Check whether the river has enough cold water for the reach you plan to fish.

Use the Mackay gauge for tailwater planning, not lower-basin assumptions.

Fish banks and riffle edges with dry-droppers during summer.

Respect private land and use official access sites when possible.

Handle mountain whitefish according to current IDFG rules.

Access & responsibility

Know the entry. Know the exit.

IDFG lists Big Lost River rules, species, access, and special whitefish language. Check the current Idaho fishing rules before fishing.

Below Mackay Reservoir

The main tailwater and flow-reference reach for this report.

IDFG Big Lost lower access

Official access-site context for lower-river planning.

BLM Big Lost River Access Trail

Public-land access context with remote travel considerations.

Upper North Fork Big Lost area

A colder and more remote upper-drainage plan with forest-road logistics.

Transparent sources

Check the facts behind the plan.

Last material review: 2026-05-31

flowRiverReports: Big Lost River below Mackay Reservoir flowUSGS monitoring location: Big Lost River below Mackay Reservoir regulationIDFG Big Lost River Fishing Planner accessIDFG Big Lost River lower access accessBLM Big Lost River Access Trail accessUSFS North Fork Big Lost River Area safetyNational 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.