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Late Summer Fishing: hitting you with some knowledge

July 21, 2010
Author: Trouts Staff

If there is one thing I have learned about fly fishing in the Rocky Mountain West, it is that nothing is constant.  River levels rise and fall, insect hatches come and go, and a trout’s mood can be Tucker-on-the-Forkinfluenced by any number of outside variables, both tangible and not.  While it would seem logical that late summer would be one of the more consistent times of year to fish, the reality is that there are evolving circumstances that can change the make up of a river and how fish will subsequently react.  Knowing how to identify these variables, as well as understanding their subsequent influence on the productivity of a stream, is a paramount skill that every fly angler in Colorado should have.  So let’s take a moment and discuss some of the key factors that influence the productivity of a river, particularly in late summer.

River Flow:  While the flow of a river is in constant flux, it is from mid July to mid September where we see water flows play such a vital role in the productivity of a fishery.  Too high and everything gets blown out leaving the fish scrambling trying to find suitable habitat.  Too low and the fish begin to get stressed because they feel more vulnerable to predators.  So it seems that we’re always looking for that elusive “prime” flow where the bugs are hatching, and the fish are happily feeding.  So how do we know when that “prime” flow is?  Unfortunately there isn’t a set standard or method to measure this; every river is different, and each will react differently to the rise and fall in flow.  But if we understand where the water that fills our streams comes from, it becomes easier to calculate when these “prime” flows will be, particularly during the late summer months.

In the west, roughly 70% of our annual precipitation comes in the form of snow.  So once the snow is gone, we are totally reliant on rainfall and cooler weather to help keep rivers at optimal levels.  This year has been unique as we started out the summer with a state wide snowpack around 100% of average.  We then quickly lost much of our snowpack in a short period of time right around the first weekend in June, when temperatures in the high country peaked in the upper 80’s and low 90’s.  As a result, we experienced river levels that peaked at near 25 year highs on certain rivers.  While these high flows are great for flushing out our streams, the mass exodus of our snowpack has ultimately left us very vulnerable to low flows and now 100% reliant on rainfall to help us get through the last months of summer.  So what does this mean for late summer fishing here in Colorado?  Pray for rain and cool weather!!!

Water Temperature:  As noted above, low river flows tend to lead to fish feeling more vulnerable to predators, and thus less willing to actively move and feed during the peak hours ofthermometer-172x300 the day.  What wasn’t discussed is the other negative impact low river flows have on the health of our fisheries.  As water levels drop in the latter part of Summer, the average daily temperature tends to rise.  Depending on what part of Colorado you’re in, these temperatures can easily reach 100 degrees or more.  While these extreme temperatures are unpleasant for us, they can be particularly problematic to the fish inhabiting our mountain streams.  Trout are a cold water fish species which require water temperatures between 35 – 75 degrees to survive; anything below or above this range can be fatal.  So as daily temperatures begin to rise in late June to early July, so does the water temperatures of all the lakes, rivers and creeks around Colorado and the west.  While it is rare that water temperatures will reach a fatal level of 75 degrees or more, it isn’t uncommon for temperatures to get hot enough where it becomes difficult for trout to be active to during the peak heat of the day.  So what does all this mean for late summer fishing here in Colorado?  Get out and fish early (i.e. from sunrise until around 11 am, or fish the evening hours from 4 pm to dark).  On most freestone rivers across the state, it is pretty standard that from the middle of July to the middle of September, fishing during the middle of the day (11 am – 4 pm) will be pretty slow.

Insect Hatches: One of the great aspects of Summer fishing in the Rocky Mountains is the abundance of insect life.  Depending on the river, you can count on seeing o_stonefly_smcaddis, pale morning duns, blue winged olives, red quills, green or gray drakes, trico’s, stoneflies, midges, and terrestrials.  It’s basically a huge trout buffet!  Yet just because there is an abundance of insect life in our rivers, it doesn’t automatically mean that it make the fishing easier.  Quite the contrary actually, it seems that the more options a fish has in terms of available food, the more difficult they can be to catch.  If a trout is keying in on caddis larvae and pupa’s, and you have a PMD (pale morning dun) imitation on your line, chances are you’re not going to be catching too many trout.  But if you put a fly on that is close to what the trout are eating, you day will quickly turn around.

It is also important to recognize when insects are going to hatch during the day.  More common than not, hatches are going to take place in the morning or evening when there is less sunlight and lower water and outdoor temperatures.  In addition, you will also see hatches following storms as both the water and air temperatures will decrease.  So what does this mean for late summer fishing in Colorado?  Have a good assortment of flies (both nymphs and dries) when you head out, do some research and find out what insects will be hatching on the river you’re going to fish, be observant when you’re on the water for any insect activity, and change your flies often until you can figure out what the fish are eating.

So the next time you’re out fishing this summer, remember these simple tips.  Nobody ever said that fly fishing was easy, but any angler would agree that the more knowledge you have about the area you’re fishing in, the better success you’ll have.

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