a year, but even prior to the drought flows have often been less than a third of that. Renegotiation The amount of water allocated was based on an expectation that the river's average flow was per year (641 m³/s). Subsequent tree ring studies, however, have concluded that the long-term average water flow of the Colorado is significantly less. Estimates have included per year (516 m³/s), per year (528 m3/s), and per year (559 m3/s). Many analysts have concluded that when the compact was negotiated, the period used as the basis for "average" flow of the river (1905–1922) included periods of abnormally high precipitation, and that the recent drought in the region is in fact a return to historically typical patterns. The decrease in precipitation has led to widespread dropping of reservoir levels in the region, in particular at Lake Powell, created by the Glen Canyon Dam in 1963, where the exposure of long-inundated canyons has prompted calls for the reservoir to be permanently drained and decommissioned. In December 2007, a set of interim guidelines on how to allocate Colorado River water in the event of shortages was signed by the Secretary of the Interior. The guidelines are described as interim because they extend through 2026, and, "acknowledging the potential for impacts due to climate change and increased hydrologic variability," interim guidelines provide "the opportunity to gain valuable operating experience for the management of Lake Powell and Lake Mead, particularly for low reservoir conditions...whether during the interim period or thereafter." The agreement specifies three levels of shortage conditions, depending on the level of Lake Mead: Light shortage. When the surface elevation at Lake Mead is below relative to mean sea level but above , the Lower Basin states will receive per year: to California, to Arizona, and to Nevada. Heavy shortage. When the surface elevation of Lake Mead is below but above , per year will be delivered to the Lower Basin states: to California, to Arizona, and to Nevada. Extreme shortage. The most severe shortage considered in the interim guidelines is when the level of Lake Mead drops below , in which event per year will be delivered to the Lower Basin states: to California, to Arizona, and to Nevada. On August 15, 2008, Arizona senator John McCain called for the compact to be renegotiated. Due to the senator's position as the Republican Party nominee in the 2008 U.S. Presidential