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Big Lost River water or watershed scenery in Idaho
All Idaho reports

Big Lost River

Photo: BLMIdaho Opens in a new tab. · CC BY 2.5

Should I go fishing?

Go with caution.

Flow is high for this time of year.

Best option: Floating. A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.

Check first: Water temperature is unconfirmed. Use a stream thermometer before fishing.

Rating updatedRiver local time:

Why this recommendation?

USGS shows 288 cfs. The flow has been stable over about 6 hours. USGS marks this reading provisional and may revise it. USGS records for this date (1904-2025, 112 readings) show a median near 211 cfs and the upper quartile near 284 cfs; today's flow is high for the date. Fishable water may exist, but do not rate it highly without a safe access, clarity, and wading or boat plan.

Available live dataHighHow much of the live flow, weather, and alert data came back.
Confidence in this recommendationMediumWater temperature not verified; do not use this score to clear warm-water risk.
Usually checks again about every 45 minutes

If you go

Check first
Use the RiverReports and USGS below-Mackay gauge for the main flow check.
Try
Check whether the river has enough cold water for the reach you plan to fish.
Leave when
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.

Flow and weather

Showing server-checked river conditions while current readings refresh.
Flow
288 cfs
From the latest stored river check. Open the official source for the newest reading.
Air forecast
53°F
Next 24h high
73.0°F

NWS air forecast near

Rain chance
0%

This is an air forecast, not a river-temperature reading. Check the water with a stream thermometer before fishing in hot weather.

Latest stored weather check

Mostly Clear

Wind
10 mph
Weather checked
More forecast and source details

Mostly Clear

National Weather Service

Big Lost River Mackay forecast point

The National Weather Service forecast is temporarily unavailable.

Live data

Where these readings come from

RiverReports gauge: Big Lost River below Mackay Reservoir

Official USGS station Opens in a new tab.RiverReports gauge Opens in a new tab.Official NWS forecast Opens in a new tab.

Flies for this time of year

Seasonal starting points, not a live hatch report. Match what you see on the water.

See flies for every season
SeasonWhat to look forFlies to try

Each fly name opens its matching Fly Box guide.

More fishing tips

Methods, fish species and trip notes

How to fish Big Lost River today.

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.

Wading, bank fishing or floating?
Best access methodFloating

These scores compare the risks of wading, bank fishing, and floating.

Wading42/100

Wading is the most sensitive plan today. Use protected edges only, avoid crossings, and downgrade quickly if clarity or current feels wrong.

Bank fishingNot recommended

This report does not recommend bank fishing on this reach.

Floating · Best fit66/100

A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.

Full river report

Is Big Lost River worth the trip today? Check flow, weather, hatches, access, and rules for Idaho before you go.

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.

Fishing words used on this page
U.S. Geological Survey (USGS) gauge
A USGS gauge is an official river station that measures flow or water height.
Cubic feet per second (cfs)
Cubic feet per second, or cfs, shows how much water passes the gauge each second.
Nymph
A nymph is a young water insect. Anglers also use the word for a fly that copies it.
Riffle
A riffle is shallow, fast water broken by rocks.
Attractor fly
An attractor fly is bright or bold. It draws a strike without copying one exact insect.
Blue-winged olive (BWO)
Blue-winged olives, or BWOs, are small mayflies that often hatch in cool or cloudy weather.
Caddis
Caddis are small water insects. The adults look a little like tiny moths.
Emerger
An emerger copies an insect as it rises from below the water toward the surface.
Midge
Midges are tiny flies and a common trout food, especially in cold water.
Pale morning dun (PMD)
Pale morning duns, or PMDs, are light-colored mayflies that often hatch in warmer months.
Why this call

Why this score

FlowUse caution

USGS shows 288 cfs. The flow has been stable over about 6 hours. USGS marks this reading provisional and may revise it. USGS records for this date (1904-2025, 112 readings) show a median near 211 cfs and the upper quartile near 284 cfs; today's flow is high for the date. Fishable water may exist, but do not rate it highly without a safe access, clarity, and wading or boat plan.

Best mode nowUse caution

Float: A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.

SeasonHelps

Fall: Cooler water, BWOs, and lower crowds can make this a strong season.

WeatherHelps

The current NWS air forecast is about 53F, and the next 24-hour high reaches about 73F. Mostly Clear.

Public alertsHelps

No active NWS alert was returned for this forecast point.

Trip notes

Check these before you choose a reach.

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 leave

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.

Big Lost River access and rules.

Use these places as starting points. Open the access link below and check posted signs before you park or enter.

Before you go

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

Where to access the river

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.

  • Irrigation and dewatering can change fishability by reach.
  • Private land is common enough that maps matter.
  • Services are limited away from Mackay and main roads.
  • Use IDFG and BLM sources for access instead of relying on old reports.

If this water does not fit the day, check another report before you start the drive.

IdahoBig Wood River fishing reportA Sun Valley trout river with different access and spring-closure issues.IdahoBoise River fishing reportAn urban Idaho river report with Greenbelt access and higher population pressure.MontanaMadison River fishing reportA larger western trout benchmark when comparing freestone conditions.
See all Idaho fly fishing reports

Sources and report details

Official agencies and primary river operators come first. Secondary tools are labeled so you know what carries the most weight.

Rules and closures

Check what is allowed, where the rule applies, and whether the river is open.

Flow and weather

Check the latest river reading and forecast before you leave.

Access and land

Check public entry points, land rules, and seasonal access changes.

See all 7 checked sources

Rules and closures

Check what is allowed, where the rule applies, and whether the river is open.

Flow and weather

Check the latest river reading and forecast before you leave.

Access and land

Check public entry points, land rules, and seasonal access changes.

Source check is old. Last checked . Open the official links now and check for changes.

See something wrong? Report a source problem. Please do not send private spots or private access notes.

How this report was checked

How this report was checked

Trip guidance checked

What was checked: This Big Lost River report is maintained from RiverReports and USGS flow data, Idaho Fish and Game rule and access sources, BLM and Forest Service access references, weather checks, and Mackay tailwater planning guidance.

Live measurements and forecasts keep their provider timestamps. Rules, access, and trip guidance use the source-review date shown here. BlueStreamFly planning advice is editorial interpretation, not an agency notice.

First-hand experience: BlueStreamFly does not claim a first-hand visit or local field test for this report unless a real contributor is explicitly named and linked.

Quick questions

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.