A Fresh Look at Western Grand Canyon
Lava Dams: Lava Dam Outburst-Flood Deposits

“The lava has run down river and and shows on side cooled in fantastic shapes… Lava all through here, run down the canyon, filling the breaks … and washed out again. Hot time then!!!”
R.B. Stanton, February 27, 1890.

On your next trip to western Grand Canyon, imagine basaltic lava flowing into the Colorado River near Whitmore Canyon. Both Powell and Stanton, the first two explorers of the Colorado River, described it vividly from their imaginations. Picture the vapors, ash, and explosions as cold water quenches hot lava. The lava piles up, moving a little bit up and downstream, eventually accumulating enough to create a dam to impound a reservoir. Suppose 350 billion cubic feet of water is backed up behind that dam, extending all the way upstream to Hance Rapid. Now, it doesn’t take much imagination to realize that this dam is unstable—after all, some of its structure is rock hydrothermally shattered by interaction with water, and its abutments rest on talus—but it holds for a while. All of a sudden, something gives inside the dam or in the dam’s abutments, and the dam collapses, releasing the impounded water rapidly in a HUGE flood, moving the dam material downstream in a wall that is initially more than 600 feet high. Envision what western Grand Canyon looks like downstream right after such a flood. You and your passengers can see what’s left of these lava dams and their outburst-flood deposits after 165,000 years . Outburst-flood deposits are common in western Grand Canyon between river miles 187 and 209 .

Over the past eight years, we mapped and investigated 49 discontinuous lava-dam outburst-flood deposits between RM 185 and RM 222 (Fenton et al., 2002; 2004). Because these deposits are very similar in appearance and cannot be distinguished on the basis of location, position, and appearance alone, we collected rock samples for geochemical analyses and cosmogenic 3Hec dating, the dating technique discussed in a previous article. Deposits with common ages and common chemical signatures were grouped together and distinguished from other deposits whose ages and chemical concentrations differed. This study allowed us to determine that at least five lava dams failed catastrophically between 100,000 and 525,000 years ago in western Grand Canyon. The flood deposits, described below, are the most convincing evidence that indicates that not all western Grand Canyon lava dams were stable, as suggested by Hamblin (1994). Hamblin (1994, Figure 64d) describes the large-scale foresets in one of these coarse gravel deposits at river mile 187.5 (river left) and states that this is the most convincing evidence for catastrophic flooding in the canyon during this period of time. He was right.
Dams created by lava flows are often assumed to be stable (Howard et al., 1982; Hamblin, 1994), and although some are stable, others have failed, releasing catastrophic floods

Vince Welch