History of Streamflow and Suspended-Sediment Collection in the Rio Puerco Basin, New Mexico

Allen Gellis, U.S. Geological Survey, Albuquerque, NM 87110

The drainage area of the Rio Puerco located in central New Mexico is 7,350 mi² (19,036 km²) of which 1,130 mi² (2,927 km²) does not contribute to surface runoff (fig. 1). The Rio Puerco is intermittent through most of its length with higher elevations receiving snowmelt and precipitation runoff events and lower reaches dominated by convective rainfall-runoff events. The large aerial extent of erosive geologic units in the basin provides a large source of available sediment to the channel. Happ (1948) estimated the sources of sediment in the Rio Puerco as: 40% erosion of the existing Rio Puerco channel (bed and banks), 30% erosion in tributary channels, and 30% sheet, rill, and minor gully erosion.

Suspended-Sediment Data
Collection of suspended sediment for computation of daily suspended-sediment discharge began in the Rio Puerco basin by the U.S. Geological Survey (USGS) in 1948 (fig. 1; Table 1). During the period 1948-1956, five stations were operating on the main stem Rio Puerco and its two major tributaries, the Rio San Jose and Arroyo Chico (Table 1). At the time of this paper, only two stations are operating in the basin, the Rio Puerco at Bernardo and the Rio Puerco above Arroyo Chico. Collection of suspended sediment data and computation of sediment loads in the Rio Puerco followed USGS procedures outlined by Porterfield (1972) and Edwards and Glysson (1988). Daily samples were collected by an observer with additional samples collected during runoff events by USGS personnel. In 1995, an automatic suspended-sediment sampler was installed at the Rio Puerco near Bernardo and the Rio Puerco above Arroyo Chico. An additional, higher stage automatic sampler was installed at the Rio Puerco near Bernardo in 1997. The sediment records can be described as good from 1948 to 1972, fair from 1973 to 1992, and good with the installation of an automatic sampler in 1993. Sediment records are labeled as fair from 1973 to 1992 because few runoff events were adequately sampled in that time period.

For the period of suspended-sediment collection at the Rio Puerco near Bernardo, 1948-96, the average annual suspended-sediment load was 4.44 million tons (4.03 metric tons). The sediment yield of the Rio Puerco is moderately high compared to world rivers (fig. 2a). However, by normalizing sediment load by average annual runoff instead of drainage area the Rio Puerco has the third highest sediment concentration (fig. 2b).

Compared to the suspended-sediment loads transported at the Rio Grande near San Marcia, located approximately 52 miles (84 km) downstream of the Rio Puerco, the Rio Puerco transported 83% of the total load of the Rio Grande from 1948 to 1973 and 64% of the total load from 1974 to 1996. For the same periods, 1948-73 and 1974-96, the Rio Puerco transported 5.6 and 2.3%, respectively, of the total runoff measured at the Rio Grande near San Marcia. In 1974, Cochiti Reservoir located approximately 92 miles (148 km) upstream from the mouth of the Rio Puerco, was closed and therefore, 1974 was chosen as a break in the two time periods. The closure of Cochiti Reservoir may have reduced the upstream contributions of sediment and therefore, may have effected downstream sediment transport.

During the period of sediment collection at the five stations (fig. 1) most of the runoff (30 to 50%) occurs in August or September (fig. 3a). Rainfall events during the monsoonal period in New Mexico from July to September are typical of convective-type rainfall events. The station Rio Puerco above Arroyo Chico has peak runoff in May and is a function of snowmelt in the Nacimiento Mountains above Cuba. Thirty-one to 51 percent of the suspended-sediment load at the five stations is transported during the monsoonal period in August or September (fig. 3b).

A sediment budget for the Rio Puerco was developed using suspended-sediment data from these five stations from 1949 to 1955 (fig. 1). Compared to the Rio Puerco near Bernardo, the largest upstream contributor of suspended sediment from 1949 to 1955, is the Arroyo Chico which drains 24 percent of the basin and delivers 34 percent of the suspended-sediment load (fig. 4). The Arroyo Chico also contributed most of the runoff (52%). The highest average annual sediment yield of any station is the Rio Puerco above Arroyo Chico (2,721 tons/mi²). The highest total sediment concentration of any station, reported as total suspended sediment for the period divided by total runoff, is the Rio Puerco above Arroyo Chico (190 tons sediment/acre-feet runoff). The Rio San Jose at Correo reported the lowest values on sediment transport of any station (fig. 4). This low value of suspended-sediment transported at the Rio San Jose near Correo relative to the main stem Rio Puerco and Arroyo Chico may reflect differences in geology, soils, and channel hydraulics.

Trends in Suspended Sediment
Suspended-sediment loads and average annual suspended-sediment concentrations show a decrease for the period of record at Rio Puerco near Bernardo, Rio Puerco above Arroyo Chico, and Arroyo Chico near Guadalupe (fig. 5). Gellis (1992) reported that this decrease was due to channel changes over time referred to as arroyo evolution. In the arroyo evolution model systematic changes in channel geometry occur following channel entrenchment, from channel deepening to channel widening. Channel widening leads to less erosive flows, increased areas on the floodplain for colonization of vegetation, and channel aggradation. The increase in sediment deposition over time leads to a decrease in suspended-sediment loads. Similar decreases in suspended-sediment loads were observed in the Colorado River basin (Gellis and others, 1991) and in the Rio Grande (Gellis, 1992). Love (1997) concluded that arroyo evolution, which is largely based on a headward erosion model, may not be applicable in the main stem Rio Puerco.

Elliott (1979) distinguished downstream channel reaches from upstream reaches based on multiple discriminant function analyses of selected channel geometric, sedimentologic, and planimetric variables. Upstream channel reaches had large width-to-depth ratios, contained relatively small amounts of silt and clay sized material in the channel perimeter, contained low vegetation density, and a lateral shifting channel that was actively eroding. The channel in the downstream reaches had relatively small width-to-depth ratios, large amounts of silt and clay sized material in the channel perimeter, high vegetation density, and a relatively stable channel position. According to Elliott (1979), the 1930's lower Rio Puerco channel was similar to the 1977 upstream reaches and led Elliott to conclude that channel stabilization was progressing from downstream to upstream reaches. Resurveys of the 1977 cross sections in 1994 to 1997 by Elliott and others (1998), reaffirmed this earlier hypothesis. Channel changes were continuing in the upper reaches of the Rio Puerco where decreasing width-to-depth ratios were observed.

Love (1997) attributed the decrease in suspended-sediment loads at the Rio Puerco to a decrease in annual peak flows since the 1930's (fig. 6). The decrease in peak flows coupled with the planting of tamarisk led to an increase in vegetation on the floodplain. The increased vegetation led to an increase in roughness, increase in sediment deposition, and a decrease in suspended-sediment loads. Further research may indicate whether the decrease in peak flows is due to climate (rainfall and rainfall intensity) or to changes in channel cross-sectional and planform geometry.

Another possible explanation for the decrease in suspended-sediment loads may include successful land-management treatments in reducing erosion implemented by various land-management agencies in the Rio Puerco basin. The Bureau of Land Management, National Resource Conservation Service, Bureau of Indian Affairs, and other agencies have been implementing programs to reduce erosion and improve vegetation cover in the Rio Puerco since the 1930's (Burkham, 1966; Soil Conservation Service, 1977). However, the success of these programs is often not monitored and quantified. The lack of monitoring of erosion-control structures has not been limited to the Rio Puerco but is a problem present throughout the Southwest. For example, a lack of project documentation, monitoring, and evaluation of watershed and riparian treatments was documented for the U.S. Forest Service southwestern region (Ahlborn and others, 1992). Gellis and others (1995) noted a similar lack of project documentation, maintenance, and monitoring for erosion-control structures built on the Zuni Indian Reservation, New Mexico.

Conclusions
Compared to world rivers, the Rio Puerco basin in central New Mexico transports one of the world's highest average annual sediment concentrations. Compared to suspended-sediment loads transported at the Rio Grande near San Marcial, the Rio Puerco transported 83% of the total load from 1948 to 1973 and 64% of the total load from 1974 to 1996. The largest contributor of total suspended-sediment load in the Rio Puerco basin is the Arroyo Chico, which drains 24 percent of the basin and delivers 34 percent of the suspended-sediment load. The highest average annual sediment yield and the highest total sediment concentration, 2,721 tons/mi² and 190 tons sediment/acre-feet runoff, respectively, was measured at the Rio Puerco above Arroyo Chico.

A decrease in suspended-sediment loads over time is observed at three stations in the Rio Puerco with long periods of record, the Rio Puerco near Bernardo, the Rio Puerco above Arroyo Chico, and the Arroyo Chico near Guadalupe. The decrease in sediment loads may be due to changes in channel and planform geometry of the Rio Puerco or to a decrease in peak flows. Both explanations favor an increase in vegetation, which leads to an increase in channel roughness and an increase in sediment deposition. It is also possible that the decrease in sediment loads is due to successful upland erosion-control strategies implemented over time by various land-management agencies. The success of many of these strategies has not been monitored or quantified.

References
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U.S. Geological Survey
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