Rapidan and Robinson Macro-invertebrates

Macroinvertebrates are useful indicators of the health of rivers and streams. Macroinvertebrates are small organisms without a backbone and include dragonfly nymphs, worms, snails, beetles, leeches, mayflies, caddisflies, small crustaceans, and other insects.

Macroinvertebrates respond to many kinds of pollution and physical disturbance to the landscape. The advantages of using macroinvertebrates as an indicator of river health are:

The Izaak Walton League Save Our Streams protocol was used by the StreamSweepers to rate the health of the macroinvertebrate community. This protocol categorizes these organisms into three categories, pollution sensitive, somewhat pollution tolerate (or somewhat sensitive), and pollution tolerant. Higher scores are provided for organisms that are more sensitive (indicating that water quality, food, cover, etc. must be favorable for them to live).

Based on presence/absence of different types of organisms, a score is totaled for each sample. The qualitative water quality ratings for numerical scores are shown below:

>22 Excellent
17-22 Good
11-16 Fair
< 11 Poor


StreamSweepers were able to collect samples near Banco (the start of the 2014 Robinson work) and Locust Dale (toward the end of the stretch).

Banco Area Sampling

The following pollution sensitive organisms were found:

The presence of these organisms means that at least for the duration of their lives, habitat was favorable. Other organisms found that are somewhat tolerant were:

The total average score for samples from this area was 22, indicating excellent water quality for this area of river.

Locust Dale Area Sampling

All of the pollution sensitive organisms, with the exception of Riffle Beetles, were found near Locust Dale. However, fewer somewhat pollution tolerant organisms were found.

The total score for this sample was 16, indicating fair water quality.

The quantity of mayflies, stoneflies, caddisflies, and other macroinvertebrates sensitive to ecological conditions was more than sufficient to indicate that the portion of the river assessed in 2014 has ecological conditions that allow macroinvertebrates to thrive. However, it is noteworthy that the quality of the samples declined as the river progressed, with the samples taken at the upper points of the river containing higher populations of sensitive species than samples taken downriver. The reason for this may be that the portion of the Robinson River that was tested has more riffles in the upper section than the lower section (the cobbles and oxygenated water found in riffles is best for macroinvertebrates).

It is also interesting that no crayfish were found in any of the Robinson River samples. The habitat in many areas appeared to be excellent. Sweeper Cole Rives theorized the following:

The best solution I could find was the point that the Robinson is generously populated by smallmouth bass and other fish species that predate avidly on crayfish. Another would be the fascinating diversity of larval dragonfly forms. Amongst the dragonfly larvae, both in the seine-net samples, in the river, and on tires or other pieces of trash hauled out of the river, were found species with legs that were stocky, short, and muscular, and legs that were long and of a fragile-looking thinness, species possessing abdomens that were short and rounded, abdomens that were enlarged and flattened, and evidence, based on a molted exoskeleton, of a species with an abdomen that is long and tapered. Additionally, in one of the samples taken I had the pleasure of finding and identifying an alderfly larvae and free-living caddisfly, both of which I had previously only been familiar with through descriptions in books.