Time series plot of past Colorado River flows created by analyzing tree-rings. The plot displays the 25-year running average of the reconstructed flows. Flows are plotted as a percentage of the 1906–2004 average of observed natural flows (18.5 billion cubic meters or 15.0 million acre-feet). The red horizontal line is the lowest 25-year running mean of observed flows (1953-1977).
Instrumental record of Colorado River streamflows. The naturalized flow of the Colorado River has varied between 5.3 and 24.0 million acre-feet per year at Lees Ferry near Page, Arizona. Naturalized flow is an estimate of the River's flow that would occur in absence of human diversions or withdrawals. One acre-foot is equal to approximately 326,000 gallons.