Channelization in the headwaters of Elephant Butte Reservoir began in the 1950s when severe drought conditions and saltcedar infestation resulted in the river channel becoming disconnected from the reservoir pool.
River disconnection has always been an issue at the headwaters of Elephant Butte. The contributing factors for the occurrence of disconnection are many: the valley slope is very slight, the incoming sediment load is high, the clay deposits are highly cohesive, and vegetation growth is extremely aggressive. During dry climactic periods when the reservoir pool decreases rapidly, all of these factors make it difficult for the river channel to cut its own channel to the reservoir pool.
With the channelization work 1950s and the 1960s, in combination with an increasing reservoir pool, the river was able to maintain a connection of its own until the early 1990s when New Mexico entered another period of dry conditions, resulting in the construction of two temporary channels. During a routine flight in 1998, it was noted that the river had once again become disconnected from the reservoir pool, so design began immediately on a third channel.
Construction Progress Fiscal Year 2000-2001
In the summer of 2000, construction began on what has been termed the Elephant Butte Temporary Channel 2000. By the time spring runoff 2001 occurred, roughly 1.5 miles of the designed 7 mile channel had been constructed to the design width of 250 ft. and the design depth of 2 ft. Roughly half a mile of the original design was dispensed with as a well-defined single channel was already in existence at the upstream-most and of the project. Construction was slow going as the conditions were less than desirable: permits required that the work be accomplished in dry or non-flowing water conditions, the saturated clay deposits caused the equipment to become stuck, and the age of the equipment (mostly ten years or older) resulted in numerous and lengthy breakdowns.
During spring runoff of 2001, there was a breach in the bankline berm which resulted in a split of flows. The breach was the result of a sharp bend that had been constructed in an area where the material consisted of sugar sand. The breach area was the primary focus of the construction for the remainder of fiscal year 2001. Realignment of the channel at the breach area began, and modification of the bankline berms began towards the end of the fiscal year after a permit modification was granted.
Fiscal Year 2002
By spring runoff 2002, a channel with a width of at least 50 ft. had been constructed along the entire design length. By the end of the fiscal year, the constructed channel consisted of 1.5 miles at design width, 4 miles at 120 ft., half a mile at 75 ft., and half a mile at 50 ft. In addition, the Low Flow Conveyance Channel spillage waters that flow down the west side of the valley were connected to the 2000 Temporary Channel.
It was also during this fiscal year that a modification was sought to allow the amphibious excavators to work in flowing water. The modification was granted and the construction speed increased, but it did not decrease the amount of down time the equipment was experiencing. The total number of work days was approximately 250, and, during that time, the down time for each amphibious excavator was about 83 work days (Linkbelt 3400), 99 work days (Linkbelt 2800), 134 work days (Linkbelt 4300), and 45 work days (Cat 322).
Fiscal Year 2003
Widening the channel to the design width (250 ft.) was the continued focus of fiscal year 2003. The work continued to progress slowly due to the previously mentioned issues as well as the increasingly large quantities of material needing to be moved in order to widen the channel. Since the reservoir pool has dropped below the Narrows, the soil in upstream reaches continues to dry out which has allowed the addition of low ground pressure dozers to the construction equipment on site. The dozers are being used to move the large volume of material that makes up the bankline berms which has been increasing productivity.
In the early part of the fiscal year, roughly 3,800 ft. of the channel was widened from 100 ft. to 125 ft., 4,300 ft. was widened from 125 ft. to 150 ft., 5,400 ft. was widened from 150 ft. to 175 ft., and 3,600 ft. was widened from 175 ft. to 200 ft. This resulted in an average width along the entire channel of 180 ft. by spring runoff 2003 which exceeded the anticipated average width by 30 ft.
Fiscal Year 2004
At the beginning of fiscal year 2004, a modification was requested to add wingwall berms downstream of the connection with the LFCC spillage waters on the west side of the channel. Without such berms, constructing a bankline berm on the west side is not possible due to the saturation of the material to the west of the channel.
Once the modification is granted, construction will focus on this portion of the channel and the widening to the west which will also prevent the need to move the extremely large volume of material on the east side in order to widen the channel to design width. In the mean time, widening continues in areas where it is possible to do so.