Editor’s Note: This is the kick-off article of this column, geared towards helping our community become native to this place. Look for subsequent installments in the Wellness and Learning section.
In Glenshire, beyond the gate of the Juniper Hills subdivision, there is a well-preserved volcanic crater. When it erupted millions of years ago its rim was level, but over the millennia the rock beneath it has been rotating and the west rim of the crater has tipped down toward the Martis Valley. Several miles away, the waters of Donner Lake lap against drifts of rock and gravel that remained behind after the last glaciers melted from the Sierra approximately 15,000 years ago. And in 1966 there was a ground rupturing magnitude six earthquake in Truckee. For the next 15 years a two-foot escarpment was visible in the gravel near the south edge of Prosser Reservoir. A road crew has since turned that disturbed gravel into a drainage ditch, but below the surface the effects of the earthquake remain.
These are cool facts, but what do they mean to you? Well, the answer is, ‘quite a lot, actually. You’d be thirstier without ‘em.’ Whenever you turn on a faucet or put a local cup of coffee to your lips, chances are that the product of these facts is about to flow down your throat. In the wilderness where we live, the bowl we drink from was formed by volcanoes, earthquakes, and glaciers.
Truckee water comes from the ground. Underneath the Martis Valley there is an aquifer that has the capacity to contain roughly a twelfth of the water of Lake Tahoe. But that doesn’t mean there’s a 3.25 trillion gallon lake underneath the airport. The aquifer isn’t full to capacity, and it also doesn’t look like a lake. It’s a deposit of sand and gravel saturated with water. When the local utility district sinks a well into the ground, water seeps out of the gravel and into the well.
But how was this huge sponge formed?
Underneath the water-saturated soils lie layers of volcanic rock, and beneath those layers rest massive chunks of granite that are impermeable to water. These chunks of granite formed approximately 100 million years ago when blobs of magma rose from the depths and cooled. They are named Plutons (after Pluto, the Roman god of the underworld), and when the Tahoe Truckee area became volcanically active roughly thirteen million years ago steaming layers of molten mud and volcanic ash began to pour across them. Castle Peak and Mount Lincoln are remnants of these flows. Over approximately the last 3.5 million years earthquakes molded the granite and volcanic material into a rough basin that is deepest just west of Glenshire, and becomes successively shallower until it finally vanishes near the Highway 267 bypass bridge.
On average, the layers of granite and volcanic material begin 1100 to 1200 feet below the surface of Martis Valley. So where did all the water-saturated sediment on top of them come from?
Over the last million years periods of glaciation have pulverized the granite and volcanic rock of the Sierra. As glaciers flowed slowly down from the Sierra crest they filled the Martis basin with massive amounts of sand, gravel, mud, and boulders. The water that flows into the soup bowls of Truckee seeps out of that glacial outwash.
Many thanks to Jim Howle and Gerald Rockwell of the U.S. Geological Survey, and to Ed Taylor of Tahoe Donner Public Utility District, for their help.
Got a question about the wilds around (and sometimes within) you? Dispatch it to bioregionboy@moonshineink.com.
