Introduction

The Pacific Northwest hydrologic unit is largely comprised of the US portion of the Columbia River basin, plus the rivers which drain the coastal ranges of Oregon and Washington. The Columbia is the second-largest river in the US in mean annual discharge, and along with major tributaries such as the Snake River, it supports extensive irrigated agriculture, the largest hydropower resource in the US, and important but threatened salmonid fisheries.

Because of the tremendous climatological diversity across the Columbia River basin, reconstructing streamflow for the mainstem Columbia presents greater challenges than other basins, such as the Sacramento River or upper Colorado River, which have more homogeneous climates. In addition, trees in the northern tier of states in the western US have moisture signals which can be weaker and more seasonally complex than their counterparts further south.

Streamflow Reconstructions

In 2004, Gedalof et al. published a reconstruction of annual streamflow for the Columbia River at The Dalles, OR, which used previously collected chronologies primarily from the lower part of the basin. This reconstruction explained only 35% of the variance in the observed flow record, but was a good first step towards more robust reconstructions for the basin.

A NOAA-funded project led by Jeremy Littell, now with the USGS, has resulted in new reconstructions of the Columbia River at The Dalles using an updated and expanded set of tree-ring chronologies. The new reconstructions explain 59% of the variability extending back to 1609, and 52% of the variability extending back to 1502.

Meanwhile, Erika Wise of the University of Arizona's Laboratory of Tree-Ring Research (now at the University of North Carolina) developed new chronologies and reconstructed streamflows at three locations in the upper Snake River, in the southeastern headwaters of the Columbia. This work was completed in 2009 and was archived on TreeFlow in June 2011.