Thursday, July 7, 2011

New Water: A Trip to the Buffalo/Roach River

It's time to be looking for insects in the Rivanna River -- but it's high, fast-flowing, and more than a little off-color.  Albemarle County is being hit almost daily with powerful storms that are putting down heavy rain in various parts of the watershed: all of this ends up in the Rivanna.

So, I decided to head north this morning and take a look at the Buffalo River, or the Roach River, or the Buffalo/Roach River -- suit yourself with the name.  I was hoping to find some damselfly nymphs -- I've found them here before -- but no luck with that venture.  Still, I did find a nice little Gomphid ("Clubtail dragonfly") -- genus Ophiogomphus (in the photo above).   It's the antennae -- not the tail -- that help us ID this nymph at the stream: they look like "clubs."  Each antenna has four segments.  But the long, fat part of the antenna is segment 3: segments 1 and 2 form the base of the antenna, and segment 4 -- if it is visible at all (it usually isn't without magnification) -- is "vestigial".  Here's an annotated form of this photo, pointing out the antennae (you can actually see segment 4 if you click on the photo to enlarge it) and the wing pads, which, note, are "widely divergent".  That's one of the features that makes this an Ophiogomphus Gomphid (see Peckarsky, Freshwater Macroinvertebrates, p. 48).

I also found another Darner dragonfly in the Roach: it was climbing around -- as they often do -- in a tangle of submerged twigs and leaves.

No "clubby" antennae on this one.  The antennae on all dragonfly nymphs save for the Gomphid, are long, slender, and multi-segmented.  Here's a microscope view of the Darner antennae.

The dominant taxa today?  Netspinning caddisfly larvae -- both common netspinners (Hydropsychidae) and Fingernets (Philopotamidae).  They were crawling all over the rocks when I turned the rocks over, and submerged root balls were loaded with them.

Fingernet caddis (genus Chimarra)

Common netspinner caddis (genus Hydropsyche)

I seem to see more netspinning caddisfly larvae in all of the streams that I sample in the summer than at any other time of the year.  I'm a little bit puzzled by this since most fly fishing hatch charts have fingernets and common netspinners hatching in mid to late spring.  At the moment, I can't really explain it.

For mayflies, I found both flatheads and brushlegged nymphs -- didn't see a single small minnow mayfly!  But I've found that in this river before.  This is a brushlegged stream, and I saw some really mature ones today, some with really dark wing pads.

The brushlegged species we find in the East is Isonychia bicolor, and Knopp and Cormier state that "Isonychia bicolor hatches appear from late spring to fall on East and Midwest waters (Mayflies, p. 85).
So, we can expect to see brushlegged nymphs, both immature and mature, for the next 3-4 months.

Two other photos.  A beautiful, fully mature, flatheaded mayfly, genus Maccaffertium, and a common stonefly, genus --what else?! -- Perlesta.  (I also found baby roach-like stoneflies and a few baby "Giants").

(In the photo below, the Gomphid and the flatheaded mayfly face off in the tray!)

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