More trees will be removed at the north end of the Corrales Bosque Preserve in the weeks ahead as the Middle Rio Grande Conservancy District continues remedies for the threatened siphon pipe that delivers irrigation water to Corrales. Starting during the second week of February, the work with heavy equipment will continue through early March, according to MRGCD Executive Director Mike Hamman.

Much of the work inside the preserve involves creating a boat ramp downriver from the rock weir over the Corrales Siphon constructed last year. Safety concerns have been raised about the risk to boaters, rafters and other watercraft users as they encounter the new rapids caused by the small boulders placed all across the Rio Grande. (See Corrales Comment letters to the editor in the January 23, 2021 issue.)

To mitigate that threat, Hamman said some of the boulders nearest the west side of the river will be removed and replaced with a more gentle, smoother passage from the upriver side of the weir to the downriver side. “We’ve gone through two separate projects here designed to save the siphon from the threat posed by the flow of the river,” he explained January 28 in a riverside interview for Corrales Comment.

“That down-cutting of the riverbed [which until recent years covered the 80-year old wooden culvert] has been going on since construction of Cochiti Dam. The original project in 2016 protected about 100 feet of the exposed siphon, but then we had heavy river flows of 2019 when flows here were consistent for weeks on end. That meant the down-cut went even further so that the siphon was completely exposed at the end of that run-off season.

“So that’s when we installed the rock weir that extends clear across the active channel. We feel like we’ve protected the siphon very well now for many years to come.” He said his agency is considering another technique called “slip lining” that would insert an inner lining material that could be expanded inside the wooden pipe to support it even further.

Since the weir has been in place, MRGCD and cooperating federal agencies have observed the results to see whether the river and its sediment load are behaving as expected. One of those has been the river dropping some of its sediment on the upriver side of the boulders which is having the effort of building up a more gentle ramp. “But we did recognize that this creates a hazard for boating, wading and possibly swimming in this area because we did raise the river up about two and a half feet or so from what it was before,” Hamman pointed out. “But we know from experience that the river will continue to moderate that leading edge so that it will start to meld into a more gradual slope.

“But we realize we will have to take additional action to make it more safe. One of the things we wanted to do, after we learned what the river was going to do, was to change this part here on the river-right side [west], to make it a smoother ramp from the upriver side of the weir to the downstream side.” He said the intended slope in the riverbed would be about 70 feet wide east-west starting at the Corrales side. The detour around the boulder weir will be on the Corrales side of the river channel rather than the east side which is under Sandia Pueblo jurisdiction. “They are very concerned about trespass there.”

Hamman said a sign will be posted along the east side warning people on watercraft not to try to go over the boulders, but instead to “stay river right” and pass through the smoother slope along the Corrales side. “People of the low to moderate skill set will probably have no problems coming through here. A skilled kayaker could probably take that rapid, but there are some sizable boulders in there.

“Now, for people who wish to put in here to go downstream and may be a little nervous about coming through this passage, we’re going to take out some of these older trees that are kind of aged out —and some of them are even being undercut by the river, so about four or five trees are going to come out. That way we can have another ramp going down to the river in a gradual slope. So people who want to avoid the weir as it is now will have two options.”

Another advantage, he said, is the new ramp will make it easier for river rescue teams to launch their watercraft at the north end of the preserve. Once the new ramp is completed, probably before summer, boaters and rafters headed down river could take out at the ramp recently built north of the weir, portage across the parking lot bypassing the weir, and put in again at the new ramp.

Another component of the improvements already completed is expansion of the visitor parking area. “We know this is a very popular area. And we wanted to make sure people can use it safely. We’ve had people who backed into the canal as they were trying to leave this area, so we wanted to make it safer.” The 83 year old 1,000-foot wooden pipe has been buried under the river bed since the early 1930s but was being uncovered by chronic erosion of the channel since upriver dams were constructed, reducing sediment deposited here. When the problem was revealed more than eight years ago, the Middle Rio Grande Conservancy District implemented temporary fixes while trying to figure out what the real solution might be.

With assistance from the U.S. Bureau of Reclamation, MRGCD constructed a rock weir immediately downstream of the pipe near the west bank of the Rio Grande. “The MRGCD had previously had an inspection performed on the siphon and found that the wood pipe was in remarkably good condition with the exception of one missing wood stave in a section near the east bank,” the district’s executive director said.

The wooden culvert brings irrigation water from the east side of the Rio Grande to the west and into the Corrales Main Canal. About eight years ago, rapids began to appear where water flowed over the pipe. (See Corrales Comment Vol.XXXII, No.16, October 5, 2013 “River Bed’s Drop Disturbs Buried Irrigation Culvert.”)

The former executive director, Subhas Shah, explained to the mayor and Village Council that the uncovering of the siphon had been caused by a reduction in silt pouring into the Rio Grande after Cochiti and other dams were constructed upstream. “In 1975 when Cochiti Dam was built, we started getting less silt coming into the river, and the river bed was getting eroded. So this is what we are seeing after 38 years.” (See Corrales Comment Vol.XXX, No. 2 March 5, 2011 “So Far, River Bed Still Degrading Here.”)

The siphon is made of a series of 20-foot long by five-inch wide wood staves that are held together with steel bands to form a pipe that is approximately 900 feet long. It brings irrigation water conveyed through Sandia Pueblo under the riverbed and into Corrales.

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