From the Denver Water Mile High Water Talk blog:
Reconstructing 400 years’ worth of streamflow data require a simple tool: tree rings.
For the past 10 years, Denver Water has worked with experts from the National Oceanic and Atmospheric Association and the University of Colorado to develop a model that details when our watersheds have been dry, wet and average since the early 1600s.
To do that, scientists study trees. During dry years, trees don’t grow much, and a narrow ring forms tight to the one that emerged the year before. During wet years, when trees go through a growth spurt, trunks develop a wide growth ring.
Ponderosa pine, pinyon pine and Douglas fir trees are more sensitive to moisture than other trees, making them a reliable record of past climate cycles. Scientists at the university take core samples from those trees (samples from the South Platte River watershed date back 400 years; samples from the Colorado River watershed date to the 1400s). Then planners compare tree ring data with 100 years’ worth of recorded streamflow gage measurements. When those two data sets are paired together in a graph, the points match almost spot-on – meaning the tree ring data correlate to past streamflow. And, because tree ring information extends back hundreds of years – much longer than Denver Water’s observed records – it helps planners analyze what would happen to our water supply if any of the pre-1900 droughts reoccurred. “This tells us what has happened in the past, but it doesn’t tell us what might happen in the future with climate change,” said Steve Schmitzer, manager of Water Resource Analysis. “It helps document variability, though. With anything in science, the more good data you have, the better.”[…]
Denver Water’s documented records show that the worst drought in our watersheds occurred in the mid-1950s, with a close second in the early 2000s. But tree rings point to a different period – the late-1840s. That’s a fact Denver Water has been able to confirm with a fair amount of certainty by studying government records from the 1840s. At that time, the government sent a host of expeditions led by Army engineers across the Great Plains. Military expeditions are often a reliable source because of their meticulous record-keeping, Schmitzer said. Their records of wet years and dry years correlated to the tree ring data scientists tracked for Denver Water.