New Debris Flow at Lava Falls
Lava Falls Rapid is, at all water levels, the most severe rapid in Grand Canyon. Its severity increased markedly in the early morning hours of March 6, 1995, when a debris flow from Prospect Creek constricted the Colorado River by approximately 50 percent. For Prospect Creek, the debris flow is the first since 1963 and the largest debris flow since 1955. The changes in Lava Falls Rapid are the most significant in Grand Canyon since the 1966 debris flow in Crystal Creek.
The debris flow was witnessed by members of our Glen Canyon Environmental Studies (gces) research trip that were, ironically, monitoring past debris flows in Grand Canyon. Our gces trip arrived at Lava Falls during the morning of March 4 and camped at the sand bar about a quarter mile above the rapid on river left. Work began immediately on repeat photography of historic photographs of the rapid. Although it had been cloudy with sporadic rain for nearly a week, March 4 was clear by noon. Rainfall began at midnight March 5. Light rainfall continued steadily the following day, but scientists matched photographs and collected data on the rapid and the source areas of historic debris flows. The storm culminated in steady hard rainfall that began about 6pm and continued until after midnight. No thunder was heard.
Between 1am and 1:30, at least three members of the trip were startled by a roaring sound that came from the direction of Lava Falls Rapid. Part of the noise was identified as distinct rockfalls. Some were concerned that the river was rising with storm runoff and that boats or the camp would be threatened. Bob Webb remembers that the noise lasted 3-5 minutes and then subsided, but others thought the sound lasted much longer.
At about 2:30am, Bob Grusy got up to find rising water and put extra lines on his boat. At about 4am, Mimi Murov rose to take down the wash table that was threatened by the rising Colorado River. The rainfall had stopped by this time. Murov thought the eddy was pooled up and calm; she thought at the time that the high water was not from a Colorado River flood but instead resulted from an increased constriction downstream.
Trip members rose at 6am to clear skies and a river that was 3 to 4 feet higher than the previous night. The discharge in the river was about 18,000 cubic feet per second. The water appeared ponded, with little movement. After cleaning up the wind strewn equipment in the kitchen area, trip members hiked to the left scout of Lava Falls to view what we thought would be high water flowing through the rapid. Instead, at 7am, we saw the new debris fan and recessional flood waters in Prospect Creek. Despite the passage of about 6 hours, the new debris fan was still changing, being reworked by the Colorado River and recessional flow in Prospect Creek.
A 1,000-foot dark brown waterfall at the upper end of Prospect Canyon was jetting about 500-1,000 cubic feet per second of water into the creek channel. This waterfall sent a fine brown mist into the canyon. Flow in the creek was a dark chocolate brown, and boulders and cobbles could be distinctly heard rolling along the creekbed. The creek channel was too high to cross until about 3pm, and flow in Prospect Creek stopped after dark on March 6. Storm runoff lasted 18-20 hours.
When we first saw it, the new debris fan extended into the river to about the left edge of the Ledge Hole. The new fan extended about 100-150 feet into the river over a distance of 600 feet. The fan sloped continuously into the river with no sign of a cutbank on its edge. Photographic monitoring of the debris fan began immediately because floodwaters prevented us from getting on the new debris fan. As the morning progressed, the edge of the debris flow was cut away by about 20-24 feet, leaving an 8-foot high cutbank on the left side of the rapid. Photographers on the left side of the rapid saw large sections of the new fan fall into the rapid. Recessional flow in Prospect Creek cut two channels through the debris fan, further reducing its size.
The rapid appeared markedly different. The entry water was extremely fast. Some well-known hydraulic features, such as the Ledge Hole and the V Wave, were still present but greatly increased in size. The right lateral of the V-Wave became much stronger than the left wave. The Ledge Hole had a different shape, a sharper drop, and a stronger hydraulic than before; the slot run was not apparent. Marker rocks, such as the Domer Rock (also known as Big Bertha, the Chub…) and the Meteor Rock, and their identifying waves and holes were not visible. The large waves that used to form between the V-Wave and the Black Rock initially were very large but disappeared by the end of the day. A large, continuously breaking wave formed off of the Black Rock, and large whirlpools formed to the right of and behind the Black Rock. Floodwaters entering on the left eliminated any possibility of running left of the Ledge Hole. Boulders were heard rolling along the bottom above the sound of the rapid. Kenton Grua and Bob Grusy thought initially that the rapid was
Downstream, the former eddies on river left and right were replaced by fast-moving water. A secondary rapid formed at the Warm Springs, but its waves subsided to riffle size as the day progressed. We interpreted the secondary riffle as water flowing around and over a new island where the pool used to be; the size of the riffle probably changed as a grave/cobble bar migrated downstream into Lower Lava Rapid. By the afternoon on March 6, a run developed just to the left of the Ledge Hole.
On March 7, we had full access to both sides of the rapid and Prospect Canyon. We had a peak of 16,300 cubic feet per second in the rapid, but the rapid looked much larger. The debris fan did not change during the day. Most of the familiar features of the rapid, such as the slot run and the marker rocks, reappeared. The Ledge Hole remained slightly different and stronger than before. The breaking wave off the Black Rock was still present, and the secondary riffle remained small. The left run continued to develop and remained in a condition judged runnable. The rapid appeared much more energized than before; the former right run appeared more than likely to flip oar boats, and the wave off the Black Rock was strong enough to potentially flip motor rigs.
On March 8 and 9, normal fluctuating flows were observed in the rapid. The entire rapid had a much higher velocity. Both Grua and Grusy felt that the right side appeared as if the discharge were 6,000 cubic feet per second higher than it actually was. The entry to the right run was much faster, the right side of the V-Wave was much larger. Several large waves that previously formed between the V-Wave and the Black Rock were no longer present, but the continuously breaking wave off the Black Rock persisted. On March 9, we ran the rapid on 11,000cfs. Grusy took his 37-foot motorboat through the right run and stated the rapid was faster but may have been easier because the Big Wave did not exist. The left run consisted of passing close to the left side of the Ledge Hole and then running a haystack wave and left of the Domer Rock and hole. Grua made the trip easily in a 22-foot motor snout, although the speed of the water entering the run was measured to be 15 feet per second. Both boats came close to the Black Rock but easily missed it.
The debris-flow project had previously identified Lava Falls Rapid as the most unstable in Grand Canyon and was finalizing work on a paper on historic changes in the rapid. Because of the previously collected information, the new debris flow was easily interpreted in terms of size and recurrence interval. The most recent debris flow at Lava Falls was in 1963; the 1995 debris fan exceeded the depositional area of the 1963 flow, and eroded all the terraces deposited in 1963. The 1955 debris flow was larger; the 1995 debris flow did not exceed the stage of 1955 and created a smaller constriction. Therefore, the 1995 debris flow in Prospect Creek is the largest debris flow in 40 years and the first in 32 years.
The 1995 debris flow in Prospect Creek set several benchmarks in Grand Canyon history. The storm that spawned it was only the second winter storm since 1872 that is known to have created a debris flow (after December 1966). This debris flow is the second largest in Grand Canyon since closure of Glen Canyon Dam (after the Crystal Creek debris flow of 1966). Changes to Lava Falls Rapid are less than changes to Crystal Rapid in 1966 but are comparable with other recent events such as House Rock Rapid in 1966-1971 and Specter, 24 Mile, and Bedrock Rapids in 1989.
One other potentially significant change we observed was at 209-Mile Rapid. Granite Park Canyon had a flash flood that closed off the left channel around the island. The left lateral on the entry to 209-Mile Rapid is now stronger, which makes missing the hole on the right more difficult.