Case Study: Equivalent Widths of the Middle Rio Grande, New Mexico

Successive reaches of the Rio Grande have maintained equivalent channel widths of 50 and 250 m, respectively, over long periods of time. It is hypothesized that alluvial channels adjust bed slope to match the long-term changes in channel width. Analytical relationships show that wider river reaches develop steeper slopes. A modeling approach using daily water and sediment discharges simulates the transient evolution of bed elevation changes. The analytical and numerical models are in very good agreement with the longitudinal profile measurements of the Bosque del Apache reach of the Rio Grande, NM, from 1992 to 1999. The slope of the 50 m wide reach was 50 cm /km and the slope of the 250 m wide reach of the same river increased to 80 cm /km. This unsteady daily transient model compares well with a steady transient solution at a constant discharge close to the mean annual flow. The transient slope adjustments can also be approximated with an exponential model. Accordingly, it takes about 20–25 years for the Rio Grande to achieve about 90% of its slope adjustment. DOI: 10.1061/ ASCE 0733-9429 2009 135:4 306 CE Database subject headings: Streams; Hydraulics; Geometry; Channels; Numerical models; New Mexico; Width.