Conceptual Restoration Plan Update: Phase I
CURRENT HYDROLOGY AND GEOMORPHOLOGY
Rio Grande hydrology has an effect on all the reaches in the Task Force Project Area. It is characterized through the Project Area by a variable spring runoff from snowmelt in the upstream watersheds, summer monsoon events, and periods of channel drying in the summer and fall. Over the past century, there have been major alterations to the hydrology through construction of upstream dams on the mainstem and major tributaries, a transbasin diversion (the San Juan Chama Project), narrowing of the river channel through construction of levees and installation of jetty jacks, and increased urban and agricultural water use. Jetty jacks were installed in the early 1930s to establish and confine the river to a stable channel (USACE 2004).
Recent changes to the hydrology of the Project Area have been predominantly controlled by releases from Cochiti Dam and include the following:
A decrease in overall volume of water in the river because of drought in New Mexico that started in approximately 2000.
A decrease in the peak spring runoff flows released from Cochiti Reservoir because of concerns about levee stability.
An increase in the flows during the low-flow periods of summer and fall to support endangered species.
Cessation of Bureau of Reclamation’s (Reclamation) river maintenance due to compliance with the Endangered Species Act (ESA) changing the geomorphology of the river channel.
The drought in New Mexico has resulted in a decreased spring runoff both in magnitude and duration during most years since 2000. An analysis of flows at the Rio Grande Floodway Near Bernardo Gage (USGS 08332010), which is at the upstream end of the project area, shows how flood events have decreased in the period from 2000 to 2020 when compared to the earlier post Cochiti Dam period 1975 to 2000. The return period flows from 2 years to 50 years are all lower with the greatest percent change in the more frequent return periods.
Flood events have decreased in the 2000 to 2020 timeframe compared to earlier post Cochiti flows. However, the yearly volume of flow into the Middle Rio Grande, as measured by total flow at the Otowi Gage, has been variable with no consistent trend. The decreased peak flows result in a less dynamic channel because of decreases in bank erosion, vegetation scour on islands and bars, and sediment transport. There have been multiple years during this time period when spring runoff flows have decreased to the point at which there is limited overbank flooding or scour of vegetation, allowing the establishment of permanent vegetation. As a result, the trend through the Project Area has been a narrower, deeper channel with increasing vegetation on the bars and islands, and channel incision resulting in less floodplain connectivity. The increase in base flows during dry times to support endangered species, which typically occur during peak vegetation growing seasons, may also contribute to the increase in vegetated bars and islands.
While the Rio Grande hydrology affects all reaches in the Project Area, the other significant driver of change in the past 20 years, the pool elevation of Elephant Butte Reservoir, mainly affects downstream Reaches 4, 5, and 6. During the most recent wet period in the 1980s and 1990s, the reservoir filled, topping 2 million acre-feet of storage in May 1984, and remained close to capacity through 1998. By 2003, it had dropped to below 10 percent of capacity and has remained below 25 percent capacity, with a few minor exceptions, through 2020 to date. As the pool elevation dropped, the delta of the reservoir moved downstream approximately 25 miles, dropping the elevation of the river channel bed. The drop in the reservoir’s pool elevation and the corresponding lowering of the river channel bed elevation has caused a headcut to move upstream, resulting in incision of over 11 feet in Reach 6, incision from 11 feet to 2 feet moving upstream in Reach 5, and from 2 feet to zero in the lower part of Reach 4.
Geomorphology and Sedimentation Trends
Breaking the Project Area into individual subreaches enables the differences in channel and floodplain characteristics to be defined and individual reach restoration options to be pursued that fit these specific characteristics. The boundaries of the reaches are generally approximate as the river channel characteristics tend to change gradually. The exception is Reach 2, with definitive boundaries at the Rio Salado confluence and the San Acacia Diversion Dam (SADD). The individual reaches have been developed both by natural conditions such as underlying geology, number and size of tributaries, channel slope, vegetation encroachment, and bed and bank material size and cohesion and through constructed facilities such as levees, the SADD, the low-flow conveyance channel (LFCC), Elephant Butte Reservoir, and the dredged delta channel just upstream of the reservoir pool.
Reach 1 starts at the north boundary of Socorro County and the channel characteristics are a continuation of conditions upstream. From the county line to the Rio Puerco confluence, the channel is wider but narrows between the Rio Puerco and Rio Salado confluences. In the early 2000s, this reach had unvegetated, dynamic mid-channel bars. Since that time, the channel has narrowed as more bars have vegetated and islands have developed. Channel degradation is the prevailing trend, although it is generally limited with good channel floodplain connectivity. It is a sand bed channel. Most of the reach has levees on both sides of the river. The Rio Puerco is the major tributary in the reach, entering on the west side of the river. In addition, there are several smaller arroyos that enter from the east side.
Reach 2 begins at the confluence of the Rio Salado, which contributes substantial coarse sediment to the Rio Grande, creating a natural grade control. This influx of sand, gravel, cobble, and small boulders creates a dynamic reach downstream to the SADD pool with riffles, extensive bank erosion, channel reworking, and small channel evulsions. Bank erosion has widened the channel, creating a new lower elevation floodplain that is inundated during higher flows, but in general the historic floodplain is elevated above the channel bed, which allows for no channel connectivity. The downstream end of the reach is a backwater of the SADD. There is a levee along the west side of the reach, and one arroyo entering on the east side.
Reach 3 begins downstream of the SADD. Because of sediment being trapped behind the dam, this reach is incised with high banks and a coarse-armored bed of gravel and cobble. The effects lessen moving further downstream from the dam, but in general there is very limited floodplain connectivity. As in Reach 2, bank erosion has created a narrow lower elevation floodplain within the historical channel banks. The LFCC begins at the SADD and, while it was in operation from the 1950s to the 1980s, river flow up to 2,000 cfs was diverted from the channel into the LFCC. The resulting decrease in channel flows resulted in channel narrowing and vegetation encroachment on the banks and bars. After flows were returned to the channel the increased flow and narrower channel caused thalweg degradation, comparison of the thalweg elevations from 1990 to 2010 shows and average decrease in thalweg elevation of 5.1 feet for the six miles downstream of SADD (Tetra Tech 2011). There is a levee along the west side of the reach, and several arroyos entering on the east side.
Reach 4 begins at Escondida. Historically this reach was a wide, open channel with dynamic unvegetated bars, but the recent trend has been significant channel narrowing with bars and islands becoming vegetated, causing a loss of channel capacity and a decrease in channel width. While the overall trend of narrowing is occurring in this reach there have been multiple channel widening projects that cause the reach average to show a widening reach. Further analysis is necessary to separate the overall narrowing trend from the widening occurring at select locations. In general, the reach maintains good floodplain connectivity. The channel is predominantly sand bed, and sediment supply in the reach is high due to sediment inputs from arroyos entering from the east. The tributaries supply coarse-grained sediment with deposits that act as grade control where they enter the Rio Grande. There is a levee that runs continuously along the west side of the reach.
Reach 5 begins at the south boundary of BDANWR. The reach is a sand bed channel and is historically an aggregational reach that resulted in a wide open channel with unvegetated bars and good floodplain connectivity. However, the lowering of the pool elevation in Elephant Butte over the past 20 years resulted in a headcut moving through this reach, causing channel incision, channel narrowing, lowered groundwater elevation, and greatly reduced floodplain connectivity. Because the floodplain is generally unconsolidated sands and the headcut has dropped the river channel below the root systems of the floodplain vegetation, the banks are easily erodible, resulting in areas of rapid bank erosion and the corresponding development of small areas of new floodplain within the historical channel banks. A levee runs continuously along the west side of the reach, and several small arroyos enter from the east side at the upstream end of the reach. These arroyos do not connect to the river channel.
Reach 6 begins at the upstream end of a full pool in Elephant Butte and most of the reach was underwater through 1999. As the pool elevation dropped, Reclamation and NM Interstate Stream Commission dredged a channel through this reach to maintain connection between the Rio Grande and the reservoir; the delta channel. The reach is aggradational, so the channel needs constant maintenance dredging, which prevents any change in channel width. Spoil bank levees built from the dredge material maintain a narrow straight channel with no floodplain connectivity. The floodplain outside of these non-engineered levees has become densely vegetated. The channel is sand bed supplied by sand transported downstream from the reaches above; however, the banks and surrounding floodplain are predominantly fines, deposited when the pool was full. Sediment plugs occasionally form in this reach causing levee breaches, which inundate the surrounding floodplain. Numerous arroyos enter this reach from the west, but none of them connect to the river channel.