Geology and Fly Fishing: How Rocks Affect Populations of Trout

By Ethan Hollifield

Did you know that rocks are one of the biggest limiting factors for finding trout in a stream?

It’s surprising to many of my clients when I tell them that on trips. The geological makeup of a trout stream can significantly influence trout populations. I’ll try to explore this topic without hopefully bringing up any painful memories of your high school earth science class.

Rocks are classified into three major categories: igneous, sedimentary, and foliated/ non-foliated metamorphic rock. Most of the Southern Appalachian Mountain range is composed of igneous rock, with examples ranging from feldspar, quartz, mica, and granite. Sedimentary and metamorphic rock types are also found here, but in extremely small quantities compared to the middle Appalachians, famous for its coal deposits.

The igneous rock that is found along many streambeds in the Southern Appalachians are made up of chemical compounds that have an acidic pH in their nature. The pH level of a stream directly influences the availability of nutrients to aquatic organisms/plant life. This results in less aquatic insect life, and thus slower growth rates of trout. When you take into account natural predation and the mortality rates of fish from other factors, the result is that some trout streams have a relatively low population of fish. In some wild trout streams, it can take a trout four years to reach its reproductive maturity age, which roughly equates to a fish being around 7” long. (This is also why fly patterns and hatches aren’t as important in WNC as anglers think they are, but that’s a topic for another article).

If we were to travel to central Appalachia, say to a Spring Creek in Pennsylvania, we would see a totally different perspective. Central Appalachia has much larger deposits of particularly sedimentary rock, which has a higher pH content due to the release of compounds called “carbonates” as the water runs over the streambed. This higher pH content from these carbonates in the water results in more nutrients being available throughout the course of a watershed, which results in higher populations of aquatic insects, and higher overall trout populations as a result.

Having said this, it is important to note that there are more factors influencing trout populations, aside from it’s geological make up. Tailwaters like the South Holston/Watauga Rivers in East Tennessee and the Davidson River in Brevard are two examples of streams with artificially high rates of nutrient content. Tailwaters that are released from dam generation constantly wash in nutrients from the bottom of a lake, which creates a constant source of nutrient rich, cold water that can make for extremely high populations of fish. The Pisgah hatchery located on the Davidson River also does this in a slightly different way. Water that is pumped out of the river into the raceways of the hatchery has to be filled with an artificially high level of dissolved oxygen which, when combined with the excess nutrients created by fish waste/algae growth in the hatchery, is then flushed back into the river. This explains why the Davidson always has a relatively high population of wild trout despite its geological make up being predominantly acidic igneous rock (and also, despite the extreme fishing pressure/unfortunate amount of poaching that occurs on that river).

Studying these factors has led me to pay closer attention to the overall geological makeup of a watershed, and as a result, has led to many conclusions as to why some trout streams in the Southern Appalachians seem to hold higher populations of trout than others. I’ve barely scratched the surface on this topic, but it’s just one of the many fascinating aspects of fly fishing that one can spend a lifetime learning about.

Ethan Hollifield is a member of a conservation organization called 2% For Conservation and a guide for Southern Appalachian Anglers.

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