Summer fly fishing in southwest Colorado is genuinely spectacular – big hatches, willing fish, long days, and some of the most beautiful water in the West. It also asks more of anglers than any other season. Warm air temperatures, lower flows, and longer sun exposure on the water mean that fishing well and fishing responsibly are the same thing. The angler who understands that is going to have a better day, on better water, than the one who doesn’t. This guide covers the practical side of summer angling – water temperature, time of day, how to read conditions, when to adapt your approach, and what kinds of water and species open up when rivers get warm. None of this is complicated. It’s just worth knowing before you go.
Water Temperature is the Variable That Runs Everything
Trout are cold-blooded. Their metabolism, their immune system, their ability to take in oxygen, and their capacity to recover from stress all track directly with water temperature. In cool water, a healthy trout can absorb the stress of being caught, fought, and released and swim away without lasting damage. In warm water, that same sequence can be fatal – even when you’ve done everything right. Here is a practical framework drawn from fisheries research:
Water Temperature
Fish Condition
What to Do
Below 60°F
Optimal
Fish normally — recovery is fast
60°F – 65°F
Good
Fish normally, release quickly and carefully
65°F – 68°F
Caution zone
Fish early or late; shorten fight time
68°F – 72°F
High stress
Stop fishing for trout
Above 72°F
Dangerous
Move to warm water species
Research from Idaho Fish and Game found that trout mortality was 69% higher when fish were landed at 68°F compared to water temperatures below 65°F. More recent research from Keep Fish Wet suggests the threshold at which mortality begins rising exponentially may be lower than the commonly cited 70°F mark.
68°F is not a safe ceiling. It is a warning.
There is a useful counterpoint to this worth understanding: catch rates drop significantly as water temperatures climb. Fish that are stressed by heat are also less likely to feed actively. So the angler who keeps fishing into hot afternoon conditions is often catching fewer fish and doing more harm to the ones they do catch. The math runs in one direction earlier in the day is better for you and for the fish – think 5am to 6am start and get off the water by 12noon.
Why Low Water Compounds the Problem When flows drop, two things happen at once: the volume of water decreases, and it heats faster. Less water in the system means it is easier for water to heat; increasing the affect of solar and air temperature. It also means fish are crowded into fewer productive holding lies – deeper pools, shaded runs, cold-water inflows – where they’re already under competitive stress before an angler arrives. When dissolved oxygen drops below 6 parts per million, which happens in warm, low-volume water, trout become visibly stressed. Below 4 ppm, mortality can occur regardless of handling. The fish you catch in those conditions are already working hard just to breathe. This is not a reason to stay home. It’s a reason to pay attention to where you’re fishing and when.
Time of Day: The Single Easiest Adjustment
Water temperature on a river follows the air. It drops through the night, reaches its lowest point in the early morning hours before or around dawn, then climbs as the sun heats the water through the day. On an exposed, lower-elevation river on a hot day, the difference between 7 a.m. and 2 p.m. can be six or eight degrees and that difference is the difference between fishing responsibly and fishing at the edge of what’s safe for the fish.
The practical version of this is simple:
Fish early. Be on the water at first light if you can. The fish are often actively feeding, the temperatures are at their lowest, and you have the most flexibility for the day ahead.
Check temperature at midday. Don’t guess. A streamside thermometer costs less than a box of flies and is more useful than most of what’s in it. Check when you arrive, and check again before you continue fishing into the afternoon.
Rest the water in the heat of the day. If temperatures are climbing into the caution or high-stress range, take a break. Eat lunch in the shade, take a nap, explore a side canyon. Come back in the evening when temperatures begin dropping again after the sun leaves the water.
Evening fishing can be excellent – but be aware that fish stressed during a hot afternoon may still be recovering. Read the fish, not just the thermometer. If they’re lethargic, sitting deep and not feeding, they’re telling you something.
Reading the Water: Where Fish Go When It Gets Hot
Fish don’t disappear in warm weather – they relocate. Understanding where they go is as useful as knowing when to fish.
Deep pools hold cooler water longer because the sun takes more time to penetrate depth. Fish stack up here in summer afternoons. These spots fish well in early morning before they warm through but depending on the size of the pool and river may not be the best idea since they are already stressed.
Shaded canyon sections receive less direct solar input and stay cooler than exposed sections on the same river. Moving upstream into a tighter canyon, or finding a section with a north-facing bank, can buy you meaningful degrees.
Cold tributary inflows and spring seeps are thermal refugia – places where cold groundwater enters the main river and fish actively seek out. You’ll often find fish concentrated tight to these spots on a hot day. Fish them gently. Concentrated fish are more vulnerable to cumulative stress, and a spring seep that gets hammered every afternoon will stop producing.
Riffles and pocket water are better oxygenated than flat pools, which matters when dissolved oxygen is declining. Fish that need oxygen often move into moving water even when temperatures are not ideal. Although be thoughtful and use the info above when using this approach. If a resource has had high temps for a while don’t target stressed trout in places they rely on to get a break.
Approaching New Water Types: Expanding Your Season
One of the most useful things a summer angler can do is develop the willingness to fish different water than they normally would. When lower-elevation rivers are warm in the afternoons, water that you’ve never considered becomes the right answer.
High-Elevation Streams and Lakes
Waters above 9,000 to 10,000 feet operate on a different thermal calendar than valley rivers. Air temperatures are cooler, solar exposure is shorter on streams shadowed by high terrain, and snowmelt from residual high-country snow keeps water cold well into summer. The fish tend to be smaller like cutthroat and brook trout in remote drainages but they are willing, they are healthy, and the conditions are appropriate for angling in a way that lower-elevation water sometimes isn’t on a hot afternoon.
Access often requires hiking, which is itself part of the experience. Trips to the High Country combine genuine backcountry access with excellent fishing for wild fish in settings most anglers rarely reach.
Tailwater Fisheries Rivers that receive their flows from deep reservoir storage have a significant thermal advantage over free-flowing rivers. Cold water released from the bottom of a reservoir enters the river well below ambient air temperature, buffering the downstream reach against the heat that affects unregulated rivers. This is why tailwater fisheries are often the most consistent option through the hottest part of summer. The fish are still there, the temperatures are more moderate, and the fishing can be technical and rewarding.
The tradeoff is that tailwater fish are often well-educated and the water is clear and low. Longer leaders, lighter tippet, precise presentations, and a careful approach matter more here than on higher freestone streams.
Stillwater: A Different Discipline Worth Learning
Stillwater fly fishing is the option most river anglers overlook, and in a warm summer it deserves genuine consideration – not as a fallback, but as a legitimate and often excellent form of the sport.
Lakes and reservoirs operate on different seasonal rhythms than rivers. In midsummer, fish move into deeper, cooler water during the heat of the day, then become accessible again in early morning and evening when they move into the shallows to feed. This makes the same time-of-day logic apply, but with different techniques and a different quality of experience.
What stillwater fishing looks like in practice Early morning is often chironomid (midge) time – patient, technical fishing with slow-sinking lines and precise depth control. The takes are subtle. The fish, particularly large rainbows and browns that have been growing in a reservoir environment, can be significantly larger than what most anglers encounter in streams.
Through mid-morning and into the day, damselfly nymph and Callibaetis (speckled wing dun) patterns become effective as those insects become active. Later in the day, streamer fishing near dropoffs and along submerged structure can produce aggressive takes from fish that are now hunting. Stillwater fishing rewards observation and patience more than constant casting. Reading the surface for rises, watching for feeding lanes, understanding where wind pushes food – these are skills that transfer back to river fishing in ways that make you a more complete angler.
RIGS offers guided stillwater trips in both walk-and-wade and float formats. Float trips on reservoirs are comfortable and accessible, and a good option for groups with mixed fitness levels or mobility considerations. These trips are best suited for older kids and adults (intermediate to advanced anglers) for younger children, our pond fishing options at Ridgway State Park are a better starting point.
Warmwater Species
When afternoon river temperatures make trout fishing irresponsible, warmwater species become a legitimate and genuinely challenging alternative. Carp, the spookiest of the river fish deserve a patient intermediate to advanced angler to sight-fishing to feeding fish in shallow water. It is as technical and demanding as any trout fishing if not more, and the fish are large, powerful, and fully capable of humbling experienced anglers. Bass in lower-elevation reservoirs take streamers and poppers aggressively and provide excellent fly rod sport. Neither requires the same temperature management that trout do.
Fishing for a new species also opens up water you’ve probably driven past without stopping. That kind of exploration tends to make you a more adaptable angler over time.
Summer Rains, Muddy Water, and What It Means for Fish
Colorado’s monsoon season, typically arrives in July and runs through September. It is genuinely welcome after a warm temperatures have blanketed the region. Cloud cover shades the water, cooler air temperatures follow storm systems, and precipitation can help recharge flows that have been dropping since runoff peaked. Rain is good news for the watershed in the long run.
In the short term, however, a monsoon thunderstorm hitting dry, compacted ground creates conditions that are worth understanding before you head to the river the next morning. After prolonged periods with no precipitation, soil loses its ability to absorb rainfall quickly. Water that would normally soak in and percolate slowly into streams instead runs off fast, carrying with it fine sediment, clay particles, and surface debris. The result is the kind of muddy, chocolate-colored water that most anglers recognize as unfishable for a day or two. But the turbidity itself carries consequences beyond visibility.
Peer-reviewed research published in Conservation Physiology documents that suspended sediment particles absorb sunlight and warm surrounding water – meaning a turbid river after a summer storm can actually be warmer than it would be on a clear-water day with the same air temperature. The particles act as small heat absorbers distributed through the water column. This effect is most pronounced in shallow, slow-moving sections where suspended material stays in suspension longest.
The second problem is at the gill. That same research documents that sedimentary turbidity can clog or damage fish gills, reducing their ability to take up oxygen. The same oxygen that is already under pressure from warm water. A fish dealing with elevated temperatures, lowered dissolved oxygen, and compromised gill function simultaneously is under compounded stress that catch-and-release cannot easily offset.
What this means practically After a significant summer rain event, give the river time to clear before fishing lower-elevation sections. Muddy water is not just a visibility problem, it is a water quality problem for the fish already under thermal stress. Higher-elevation streams and tailwaters below deep reservoirs tend to clear faster and are less affected by surface runoff, making them better options in the day or two following a storm.
When flows are stained but clearing, check temperature as you normally would. A post-storm river can be cooler, warmer, or the same depending on the storm’s intensity, timing, and what elevation the runoff originated from. Don’t assume the rain improved conditions without confirming it with a thermometer.
The monsoon is an important part of the seasonal rhythm of southwest Colorado’s rivers. Pay attention to it, fish around it thoughtfully, and let it do its work when the rivers need it more than you do.
Handling Fish Well: The Fundamentals
Regardless of where or when you’re fishing, how you handle fish determines whether catch-and-release actually works.
Land fish quickly. A prolonged fight fills a fish’s bloodstream with lactic acid. Use tippet appropriate to the fish size for a faster fight is more ethical, not less sporting.
Keep the fish in the water. Ten seconds out of water is a baseline; in warm conditions, less is better. Have your camera ready before the fish comes to the net. A photo in the net, in the water, is better than lifting the fish for a grip-and-grin.
Wet your hands before touching fish. A dry hand strips the protective slime coat that shields fish from disease. Use rubber or coated nylon nets rather than knotted mesh for the same reason.
Hold the fish until it’s ready. Support it upright in the current until it kicks away on its own. If it lists or can’t maintain orientation, hold it longer. A fish that isn’t recovering normally is telling you something about conditions.
Watch fish behavior as a real-time gauge. If trout are hugging the bottom and not feeding, they may be in temperature-related stress. That’s not just a tactical signal, it’s a sign that the water may not be appropriate to fish at that moment.
The Gear That Actually Matters in Summer
Streamside thermometer – check temperature when you arrive and again at midday. Non-negotiable.
Quick Release tools, forceps or hemostats – fast, clean hook removal with the fish in the water
Rubber landing net – protects slime coat far better than knotted mesh
USGS waterdata.usgs.gov bookmarked on your phone – check gauges before you drive an hour to a river
On photography A summer fish is not a trophy moment – it’s a living animal already under stress. If you want a photo, have your phone or camera out and ready before the fish comes to the net. The best images from warm-weather fishing are taken with the fish in the net, in the water, with the angler crouched at the surface. A fish held up in warm air for a posed shot is a fish that may not recover. When water temperatures are in the caution zone or above, take it underwater or skip the photo entirely. The fish matters more than the image.
A Word on Knowing When to Stop
Some days, the most responsible thing an angler can do is not fish or fish somewhere else. That’s not a failure. It’s a recognition that the resource matters beyond the day. Guides who make that call earn trust. Anglers who make it earn the respect of the water they fish and the anglers they fish with. When water temperatures reach 68°F or above during a sustained period, it is wise to consider avoiding those waters entirely. That’s not an extreme position. It’s good stewardship expressed as a practical guideline. The West’s rivers are resilient. Trout populations can rebound quickly when conditions improve. What they need from anglers during the hard stretches is restraint and the willingness to find good fishing somewhere else until the water is right again.
When in doubt call the shop to find out where is fishing well and where temperatures are keeping us off the river and where we are moving to. Call the Shop: 970-626-4460 | reservations@fishrigs.com
