Tuesday, December 20, 2016

Notes on the Geology of Colorado Fishing

Steven Wade Veatch

A stream, as a geological agent, is one of the most powerful forces on Earth.  Many of Colorado's magnificent landscapes are the products of what streams do best—moving sediments sporadically downstream in regular cycles of erosion and deposition.  In Colorado, the Continental Divide splits streams that flow west to the Pacific Ocean from those that flow eastward to the Atlantic and the Gulf of Mexico. The sparkling streams of Colorado not only shape the landscape but also provide great fishing.  A deeper understanding of the riparian environment and geologic processes will enhance every fishing trip.

Snowmelt gives rise to Colorado's four major river systems:  the Platte, the Arkansas, the Rio Grande, and the Colorado.  Here is a quick review of those rivers.

The South Platte begins in the high country of South Park, but when it reaches the Cheesman Canyon, south of Deckers, local geology creates some remarkable places to fish.  Granite formed in the canyon under enormous pressure several kilometers below the surface and was later exposed by regional uplift.  With the erosion of the overlying rock, the granite expanded and cracked due to the release of pressure.  Gravity now causes the rock between the cracks in the granite to break loose in concentric slabs from the underlying granite body.  This process, exfoliation, results in the rounded nature of the granite outcrops in the canyon.

Granite boulders, slabs, and gravel form bars across the South Platte that dissipate the energy of the flow, producing areas of calm water and deep pools in Cheesman Canyon.  Willows grow along the banks while aspens and spruce trees grow tall, providing shade for brown trout.  Because browns are very selective in what they eat, they are hard to catch and grow to a large size.  Anglers on this river frequently use small flies, especially the pheasant-tail fly.

The Arkansas River starts in the mountains near Leadville and Tennessee Pass and flows south and east to merge with the Mississippi in the state of Arkansas.  After spring runoff has reworked sand and gravel bars, fresh gold placers can be panned on the upper reaches of the Arkansas.  As the Arkansas River flows by the Texas Creek recreation area on its way to the Royal Gorge, brown trout can be caught with caddis flies.  The Texas Creek area is also noted for deposits of rose quartz associated with pegmatite (coarsely crystalline) granite that intruded into metamorphic rocks.

The Rio Grande River has its headwaters in the San Juan Mountains and flows through New Mexico on its way to the Gulf of Mexico.  Near Creede, at Wagon Wheel Gap, the Rio Grande offers excellent fishing for browns, brooks, rainbows, and cutthroats using a prince nymph.  Cutthroat trout like slow pools that are just opposite large granite boulders.  There are several geothermal springs in the area, and excellent specimens of fluorite occur nearby.

The Colorado River drains the western slope of the Continental Divide and empties into the Gulf of California.  The major tributaries of the Colorado River are the San Juan, White, Yampa, and Gunnison Rivers.

The Gunnison River began downcutting into the Earth after a period of regional uplift 28 million years ago.  Today steep Precambrian gneiss (metamorphic rock) walls, with pink pegmatite dikes filling cracks and fissures, rise thousands of feet above the Gunnison River in the Black Canyon.  Geological processes here have produced the best fishing spot in the state.  It is the only place in Colorado where browns and rainbows grow to 16 inches in just four years.  Anglers in this area commonly use big nymphs.

A view of the Gunnison River running through 
the Black Canyon of the Gunnison. Photo used 
by permission under a Creative Commons License. 

Geologic processes have created 1,800 lakes above 9,000 feet in elevation in Colorado.  Many of these high-country lakes, called tarns, occupy the bottoms of amphitheater-shaped cirques where glaciers eroded into the mountain.  If there are enough insects to eat and the lake is deep enough for the fish to winter, there will be a population of trout.

Maroon Lake, at the foot of snow-striped Maroon Bells, 
is one of many Colorado lakes where great fishing awaits.  
Photo © S. W. Veatch.

Trout are not always easy to catch in high lakes as they feed along the edges and can be easily spooked.  Brook trout—commonly found in high country lakes, beaver ponds, and small creeks—tend to be small because they reproduce rapidly and surpass their food supply.

Trout like to cruise most of the 11,300 miles of streams in Colorado, and if anglers consider the rock and understand the role that geology plays in fishing, they have an advantage for catching trout. It is “gneiss” to know that fishing and geology can't be taken for “granite.”

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