Monsoonal moisture dropped significant precipitation into the Mill Creek and Pack Creek drainages on Tuesday, causing both waterways to flood dramatically. The sound of rushing water and the smell of silt and ash recalled last summer’s flash floods.
“Pack Creek and Mill Creek both flooded, but Mill Creek got hit harder,” said Chris Wilkowske, supervisory hydrologist for the USGS Moab field office. “I looked out the window and it was a big black cloud over the mountains.” Closer to Moab itself, the weather was cloudy and breezy but didn’t produce any rain.
Officials at the Grand Junction office of the National Weather Service said the agency doesn’t have any rain gages in the area of the storm, mostly on the west side of the La Sal Mountains. They could confirm that most of the rain fell within an hour before the storm moved out of the area.
The United States Geological Survey maintains four gage stations in the creeks: there are two in Mill Creek above its confluence with Pack Creek, and one in Mill Creek below the confluence; a new, permanent gage on Pack Creek, where it crosses under Pack Creek road, began collecting data in June. Data from these gages are available to the public online (https://waterdata.usgs.gov/ut/nwis/current/?type=flow), but recent readings carry a disclaimer: “Provisional data subject to revision.”
The uppermost Mill Creek gage reported a record maximum stream height of 10.78 feet, high enough that the value isn’t included on USGS tables used to determine the flow rate by water height. Technicians visited the upper gage on Wednesday to take first-hand measurements and check the gage to see if it was working properly. Wilkowske explained that the gage is submerged in the creek and determines the water height using a pressure sensor. If the gage gets buried with rocks or sediment, it can give an inaccurate reading.
The farthest downstream gage, which collects data below the confluence of Pack and Mill creeks, gave a peak reading of 670 cubic feet per second on July 26. For comparison, on Aug. 2 of last year, the peak reading was 960 cfs.
“The downstream gage is kind of a reality check,” Wilkowske said. “We feel pretty confident about that one, because we were out there and could measure.” Technicians hurried to the site during the flood to take first-person readings; their measurements there made them skeptical of the readings at the upper gage. However, Wilkowske also noted that peak flows can spike and then spread out over time and distance, a phenomenon called “peak attenuation.”
On Wednesday at the upper Mill Creek gage site, the technicians measured a high water mark not far from what the gage had reported: 10.4 feet. That’s still “off the charts,” meaning the tables used to find water flow based on peak water height, or “peak stage,” don’t go up that high. USGS scientists will return to the site to survey, and they’ll then use a computer model to calculate a discharge value. The data will have to be formally reviewed and verified, a process that could take months. Once verified, the USGS website will be updated.
A 2010 flood saw a height of 9.32 feet, which was verified by technicians, at the upper Mill Creek gage, which corresponded to a flow of 1,160 cubic feet per second.
“This event was about 1.1 ft higher, so I would feel confident in saying the flow was more than 1,160 cfs,” Wilkowske wrote in an email. “My best preliminary estimate would be about 1,500 cfs. That would be a peak of record for this station which has been in operation since 1987.”
The record-breaking flood certainly had significant impacts. Dana Van Horn, manager of the Grand Water & Sewer Service Agency, said that when the creek started to flood, the infrastructure used to divert water from Mill Creek to Ken’s Lake quickly filled with mud, completely blocking flow into the lake. Water was still reaching the reservoir, however, via a natural wash that filled and began to flow into the lake, bringing mud and debris. Van Horn said she’s never seen the wash flood so substantially before.
Mud suspended in the lake flows into GWSSA pipes that deliver water from Ken’s Lake to people who use it for irrigation. Van Horn said that while GWSSA’s pipes are large in diameter and can likely handle the muddy water, sprinklers downstream can be clogged by the fine silt. The agency shut off the outlet from the lake at around 7:30 p.m. on Tuesday. On Wednesday morning, GWSSA isolated a section of its system.
“Operators started the irrigation wells and opened the drain valves. The muddy water was returned to the creek and the system was running clear and pressurized from the wells before lunchtime,” Van Horn wrote in an email.
GWSSA used a newly established community engagement platform to update its customers on the progress of the maintenance. “The platform worked well for us today,” Van Horn wrote on Wednesday. “We are grateful for the quick actions of our staff and our wonderful customers who are always so understanding when situations arise on the irrigation system or with the lake.”
Meanwhile, clearing mud out of the diversion will require heavy equipment, and access roads to reach it may not be passable right now. Van Horn said that while it’s not ideal for muddy flood water to be directed into the lake, the diversion can help prevent flooding in the city by reducing Mill Creek’s flow in the creekbed before it reaches town. The agency will get the diversion infrastructure cleared as soon as possible.
While the most significant flooding was in Mill Creek, Pack Creek also swelled above normal levels. Black soot, evidence of last June’s Pack Creek Fire, churned in the high water. The Pack Creek gage readings should be more reliable, as the device is mounted to a bridge rather than submerged. The creek reached a height of 9.49 feet; there’s no corresponding flow reading for that height.
“We will need to do the same kind of survey work to get a flow value at that station,” Wilkowske said. He’s unsure if the water height reading is correct there; technicians who visited the site thought it was unlikely the flow was that deep in the channel. However, if the reading is correct, Wilkowske estimates that the flow was probably around 180 cfs.
Higher up in Pack Creek, U.S. Forest Service Hydrologist Daniel Lay checked on the status of nets that had been installed across the creek channel to mitigate erosion. The nets were a collaboration between the Forest Service and the nonprofit Southern Utah Wilderness Alliance. Volunteers spent a day this spring helping to install the metal cable nets, which are intended to trap debris and reduce the slope of Pack Creek, thus reducing the creek’s speed and how much it erodes sediment and further incises the channel. Lay said the nets are working as intended.
“We had a debris flow a month or so ago; it held really well and there’s five feet of accumulation behind the net,” Lay said, adding that that’s a significant amount of material. One of the nets was damaged and debris escaped, bringing the accumulation down to two feet.
On Wednesday, more rain caused another small spike in Pack Creek’s flow, and Lay and other Forest personnel went to the net site to watch the debris flow; it accumulated behind the nets as expected.