~Jefferson River Montana~

The Jefferson River Flooded last weekend. All the hay fields in the area were flooded and roads that have not been submerged in recent history became gentle rivulets. The High water is good for the Jefferson River because it has had many years of below average flows–and fine sediments have built up among the interstitial spaces were many invertebrates that are important fish forage live.

When Snorkeling in recent years, I have found that if you pick up a cobble, there is beneath it only sand. Typically, you might expect to find more cobbles under cobbles. Among the sand grains are the burrows of several sediment dwelling invertebrates (e.g., Hexagenia and Ephoron sp.). According to locals, the river once supported impressive hatches of large stoneflies (Hesperoperla pacifica, Pteronarcys sp. etc).

When sediment fills in the the area under the stones, much of the habitat used buy large stone flies is lost because the interstitial spaces are simply too small for them to pass through. This results in smaller populations (of large invertebrates), and ultimately smaller hatches. It may also reduce the forage available for fish. For example, it appears that sedimentation of the Jefferson River may have caused Burrowing dragonflies (Gomphidae) to replace many of the large predatory stoneflies typically expected. We sampled the Jefferson River several years ago and found about 2-3 gomphid dragonflies per square foot and about 0.3 large stoneflies in the same area–the dragonflies were nearly 10x times as abundant as the stoneflies.  We are hopeful  that high river levels will scour sand from interstitial spaces and improve survival of stoneflies. This could result in an improvement in fishing a few years down the road…

Aside from benefiting river ecology, another effect of the flood is that pools and back waters that have not been flooded for years have now been nicely inundated for a week or more. Mosquito eggs can remain dormant for several years–until they become wet.  We found hundreds of thousands of mature mosquito larvae and pupae among the grassy ditches and fields last weekend. There were so many larvae, that in just a few minutes we observed hundreds of larvae washing across the road in shallow riffles (picture below).  By this weekend or early next week the Jefferson Valley will likely be swarmed by endless squadrons mosquitoes… and it has been such a nice spring.  If you can tolerate the bites and buzzing it might be a nice weekend to try mosquito-mimic flies… and if you have a friend who owns a hay field… maybe see if you can get permission to fish it!

mosquitoes float across the road

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