2008 Brook Trout Survey

Brook Trout Surviving in McNabb Creek

Jim Herrig  –  June 2008

The aquatic community of McNabb Creek has suffered since the late 1970’s when construction of the Tellico-Robbinsville Road exposed an extremely acidic rock formation known as Anakeesta.  Initial efforts to mitigate the effects of the acid were unsuccessful and McNabb Creek became a sterile stream devoid of fish and most aquatic invertebrates.

By the early 1990’s signs of recovery began to appear.  Hemlock Creek, an adjacent stream which had also been impacted by the acid rock, recovered to the point that it was supporting six species of fish and a diverse aquatic macroinvertebrate community. McNabb Creek had occasional rainbow trout and creek chubs present but no indication of reproduction.

In March, 2005 fine limestone sand was dumped into the headwaters of McNabb Creek to neutralize the acidic condition and allow fish to successfully spawn.  Southern strain brook trout were introduced into McNabb Creek in August, 2005.  They persisted into 2006 but no reproduction was documented.  Surveys in 2007 failed to document that the brook trout were still present or reproducing.  Rainbow trout, brown trout and creek chubs were present and reproducing.  On June 5, 2008 a 4 inch brook trout was caught and released in McNabb Creek about two miles above the confluence with the North River.  This fish was probably spawned in McNabb Creek and is confirmation of the persistence of southern strain of brook trout in McNabb Creek.

While finding brook trout in McNabb Creek in 2008 is very exciting news, the conditions in the stream are far from ideal.  A chemical survey of McNabb Creek and its major tributaries in April, 2008 revealed frightening pH levels.  In the drainage ditch that carries water off the Tellico-Robbinsville Road the water had a pH of 3.7.  As low as this reading is, it is correctable.  The limestone sand placed into the stream in 2005 appears to still be functioning because below the point where this culvert blends with the main channel, the pH had recovered to 4.5 and further downstream the recovery continued.  However, one tributary that enters McNabb Creek about 1.5 miles downstream from the headwaters had a pH of 3.9!

A new acid problem has emerged in McNabb Creek.  

The exposure of the Anakeesta formation along the Tellico-Robbinsville Road can be mitigated.  As water runs off, it becomes acidified and drains into roadside ditches.  At key points, the ditches are diverted through culverts to the downstream side of the road.  We place limestone sand into the culverts.  When it washes out it piles up below the culvert and all runoff passes through it and is somewhat neutralized before joining with the main McNabb Creek channel.

However, the acidic runoff has compromised many of the culverts by rusting them out.  The consequences of the leaky culverts are both an environmental disaster and a severe safety concern.  The rain runoff enters the culverts and leaks through the culvert into the fill slope.  The fill slopes in this area are composed of crushed Anakeesta rock.  The water re-emerges up to one-half mile down slope of the culvert outfall.  We cannot treat water this far from the road with the limestone sand.  More frightening than the acidified water is the potential impact to the road prism from water entering the fill slopes.  The weight and lubricating action of the rain water entering fill slopes could result in a catastrophic failure of the fill slope.

This potential problem has been brought to the attention of the Tennessee Department of Transportation and a simple, relatively inexpensive solution is available – place plastic liners into the leaky culverts.  TDOT may implement this corrective action soon.  Once the liners are installed we will dump limestone sand into the key culverts and hopefully, watch the pH values return to more normal levels.  The brook trout will begin to dominate the aquatic community of McNabb Creek and provide the public with an opportunity to fish for this uniquely beautiful fish in a quiet, restful setting.