Late in the Spring, I was camping at McGee Campground above Crowley Lake. I like this campground since it does not have the crowds of Convict Lake and the sites are wider. There's a nearby trailhead that takes you to McGee Pass and drops into Fish Creek and Cascade Valley. It's great scenery as well as great fishing. A caravan of white vans came into the campground, filled with students from Pomona College. The instructor had the group gathered around for a lecture on the geology of the area as he pointed to distant features of the mountains nearby. I recalled being with a similar group of botanists from UCSB when we toured California many, many moons ago.
Fortunately, I found a site on the internet that details the field trip I encountered. http://geology.csupomona.edu/docs/sierra.html
This entire area is the basin of a giant volcano, known as the Long Valley Caldera. Within this caldera are many of the volcanic features that we see as we fish but never knew the background information as to how it came to pass. We do know that there are more fish per mile on Hot Creek than any other stream in California. And volcanism plays a major role in creating the conditions for the weed growth and organisms that inhabit that stream, as well as provide food and shelter for a great number of fish. Crowley Lake is another powerhouse for fish growth, often attributed to the alkaline soils that surround the lake which provide nourishment for Chironomids, scuds, and other trout forage. The Upper Owens River is another destination spot for many of us to fish in "Spring Creek" conditions, year-round. The river originates from the waters that spring forth from the lava layers at Big Springs and run continuously throughout the year. The deep undercut banks holding large Browns often will collapse huge sections of ash-laden banks into the water during the Spring run-off as it meanders through Long Valley. All of these great fishing areas are within the Long Valley Caldera.
This caldera was formed about 760,000 years ago when an eruption that was 2,500 times larger than Mt. St. Helens took place. (I was at St. Helens when it erupted and it's very difficult to imagine something 2,500 times greater than that!). The eruption partially emptied the magna chamber within Long Valley and it collapsed on itself creating a caldera 10 miles long and 20 miles wide.