Wade4/100
Wading is the most sensitive plan today. Use protected edges only, avoid crossings, and downgrade quickly if clarity or current feels wrong.
All Idaho reports
Fly fishing report · West
Big Lost River
Is Big Lost River worth the trip today? Check flow, weather, hatches, access, and rules for Idaho before you go.
See the flow chart ↓Photo: BLMIdaho Opens in a new tab. · CC BY 2.5
Good day to fish?Latest river check
Poor
Not today. Make another plan.
39
Best option: Float. A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.
See update time and confidence
Live data checkHighHow much of the live flow, weather, and alert data came back.
Today's callHighA verified hard stop is active, so the avoid-or-wait decision is clearer than the fishing score itself.
Score calculated Aug 22, 2026, 8:39 PM MDTUsually checks again about every 45 minutes
Best access methodFloat
These scores compare the risks of wading, bank fishing, and floating.
Bank / edgeNot recommended
This report does not recommend bank fishing on this reach.
Float · Best fit19/100
A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.
Red Flag Warning issued August 22 at 1:41PM MDT until August 22 at 9:00PM MDT by NWS Pocatello ID
Your plan
Today's Big Lost River plan.
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.
- 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.
Live river check
What the river is doing
Check the latest flow and weather before you go. A quick change can make the river harder or less safe to fish.
Showing the latest stored river check while current readings load.
- Flow
- 321 cfs
- From the latest stored river check. Open the official source for the newest reading.
- Air now
- 74°F
Current NWS forecast period
- Next 24h high
- 79.0°F
NWS air forecast near
- Rain chance
- 25%
For this forecast period
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
Chance Showers And Thunderstorms
Chance Showers And Thunderstorms
- Wind
- 6 mph
- Weather checked
Recent flow chart is loading
The latest stored reading is shown above. Check the official source below if you need the newest value now.
More forecast and source details
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
How to fish it
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.
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
Public alertHurts
An active alert is in effect: Red Flag Warning issued August 22 at 1:41PM MDT until August 22 at 9:00PM MDT by NWS Pocatello ID. Check public safety sources before going.
Best mode nowHurts
Float: A float can fit better than wading only if launches, shuttle, boat skill, wind, and local rules all check out.
Short-term weatherUse caution
The forecast has storm or heavy-precipitation risk, so timing and access matter more than the score alone.
FlowHelps
USGS shows 321 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, 113 readings) show a typical middle range of 195 cfs to 458 cfs. Flow is within the typical range for this date, so weather, temperature, and access become the next checks.
SeasonHelps
Summer: Caddis, PMDs, stones, and terrestrials matter, but heat and irrigation flows decide the day.
Trip notes
Trip notes
Check these before you choose a reach.
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.
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.
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.
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.
What to try
Big Lost River flies through the year.
Match the time of year, then let rising fish and current speed guide the next change.
SeasonWhat to look forFlies to try
Spring
What to look forBWOs, midges, caddis
Early summer
What to look forPMDs, caddis, golden stones
Late summerCurrent
What to look forTerrestrials, caddis, small mayflies
Fall
What to look forBWOs, midges, October caddis
Each fly name opens its matching Fly Box guide.
Access and safety
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. 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.
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.
River sources
Official Big Lost River sources.
Official agencies and primary river operators come first. Secondary tools are labeled so you know what carries the most weight.
How this report was checked
How this report was checked
Reviewed byBlueStreamFly River Review Team
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.
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.
What this source covers
Open the current rules and check the river section, season, and special limits.
Flow and weather
Check the latest river reading and forecast before you leave.
What this source covers
Open this page and check the latest safety advice before your trip.
What this source covers
Open this river reading to check the latest level, flow, and update time.
What this source covers
Open this river reading to check the latest level, flow, and update time.
Access and land
Check public entry points, land rules, and seasonal access changes.
What this source covers
Open the land or access page, then check signs before you park or enter.
What this source covers
Open the land or access page, then check signs before you park or enter.
What this source covers
Open the land or access page, then check signs before you park or enter.
Quick answers
Quick Big Lost River answers.
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.