# Colorado River at Lees Ferry, CO

This latest version of the reconstructed flow for the Colorado at Lees Ferry was generated as part of a project supported by the California Department of Water Resources (CDWR). This project also includes reconstructions of southern California water year precipitation (San Gabriel Dam, Lake Arrowhead, Ojai, and Cuyamaca) and streamflow (Arroyo Seco and Santa Ana River) and the Kern River streamflow. This version of the Lees Ferry reconstruction was developed by Dave Meko, Erica Bigio, and Connie Woodhouse in 2017, using a set of updated tree-ring collections from eight sites in the Colorado River basin. Two reconstructions were generated: one highlighting skill, with a start date of 1416, and one emphasizing length, with a start date of 1116. The most skillful reconstruction is comparable in skill to the reconstructions from Meko et al. 2007 and Woodhouse et al. 2006 over similar time periods.

**Methods**

Average discharge of the Colorado River at Lees Ferry for the water year (October-September) was reconstructed using a two-stage regression procedure. Tree-growth at each site was first converted into an estimate of discharge by stepwise regression of discharge using tree-ring width indices, from the current year and lagged one year, as predictors. Squared terms on the tree-ring predictors were also included in the regression to allow for possible curvature in relationships between tree-growth and discharge. In the second step, the gage reconstruction was generated by averaging an appropriate set of single site reconstructions. Final estimates of discharge were interpolated from a piecewise-linear smoothed scatterplot of the observed discharge values and the discharge estimates averaged over the individual tree-ring sites. The procedure was repeated for subsets of tree-ring chronologies with different periods of common time coverage to build a “most-skillful” reconstruction, starting in the early 1400s, and a “longest” reconstruction, starting in the early 1100s. Details of the reconstruction method can be found here.

Statistic |
Most Skillful: Calibration |
Most Skillful: Validation |
Longest Model: Calibration |
Longest Model: Validation |

Explained variance (R2) | 0.79 | 0.58 | ||

Reduction of Error (RE) | 0.80 | 0.55 | ||

Standard Error of the Estimate | 18.41 MAF | 27.84 MAF | ||

Root Mean Square Error (RMSE) | 19.45 MAF | 29.29 MAF |

** Note: ** The statistics listed in the table represent average accuracy, while the reconstruction method yields error bars that vary in width over time -- generally wider for wet years than for dry years. The listed statistic R2, the decimal proportion of variance explained by the reconstruction in the calibration period, is computed directly from the reconstruction residuals. For an explanation of these statistics, see this document.

**Figure 1. ** Scatter plot of observed and reconstructed Colorado River at Lees Ferry annual stream flow, 1906-2014. Note that the R2 value here is slightly different than in the table. The table R2 value is the average explained variance from the three models that make up the most skillful reconstruction. The value in the scatter plot reflects the explained variance for the two models that cover the instrumental period (more details).

**Figure 2. ** Observed (gray) 1906-2014, and reconstructed (blue) 1900-2015, Colorado River at Lees Ferry annual stream flow. The observed mean is illustrated by the black dashed line.

**Figure 3. ** Reconstructed annual flow for the Colorado River at Lees Ferry (1416-2015) is shown in blue. Observed flow is shown in gray and the long-term reconstructed mean is shown by the black dashed line.

**Figure 4. ** The 10-year running mean (plotted on final year) of reconstructed Colorado River at Lees Ferry annual stream flow, 1416-2015. Reconstructed values are shown in blue and observed values are shown in gray. The long-term reconstructed mean is shown by the black dashed line.

**Figure 5. ** Scatter plot of observed and reconstructed Colorado River at Lees Ferry annual stream flow, 1906-2014 (more details).

**Figure 6. ** Observed (gray) 1906-2014 and reconstructed (blue) 1900-2015, Colorado River at Lees Ferry annual stream flow. The observed mean is illustrated by the black dashed line.

**Figure 7. ** Reconstructed annual flow for the Colorado River at Lees Ferry (1116-2014) is shown in blue. Observed flow is shown in gray and the long-term reconstructed mean is shown by black dashed line.

**Figure 8. ** The 10-year running mean (plotted on final year) of reconstructed Colorado River at Lees Ferry annual stream flow, 1116-2014. Reconstructed values are shown in blue and observed values are shown in gray. The long-term reconstructed mean is shown by the black dashed line.