DOWNSTREAM EFFECTS OF COCHITI DAM ON THE MIDDLE RIO GRANDE AND THE CHANNEL TRANSITION ZONE NEAR ALBUQUERQUE, NEW MEXICO

The middle Rio Grande system of New Mexico has undergone major adjustments since construction of Cochiti dam in 1973. We studied spatial and temporal changes over a 25.5 km reach from Bernalillo downstream to Albuquerque. Prior to dam closure, bed sediment median grain size (D₅₀) was nearly uniformly fine sand. Channel morphology was characterized by a multi-thalweg, shallow, bar-braided planform, with uniform channel width due to bank stabilization structures and associated dense vegetation. After dam closure, a zone of channel incision and bed coarsening rapidly developed and migrated downstream at up to 5 km/yr (Lagasse, 1980). This rate has subsequently slowed and has averaged 0.7 km/yr since 1980, based on the current location of the coarsened bed. The transition zone defined by coarsened bed sediment now lies within the middle of the study reach and is bounded by two major tributary arroyos, which may episodically contribute large volumes of sandy sediment. Upstream of the transition zone, the channel planform is island- to bar-braided with a dominantly single-thread, narrow and deep active channel (width/depth ratio: 50-90). Bed sediment upstream of the transition zone is dominated by pebble and cobble gravel, with sand within the channel banks and in isolated lenses within high-flow channels. Upstream D₅₀ values ranged between 64 mm and 1 mm, whereas in the transition zone values range between 32 mm and 0.125 mm. Downstream of the transition zone the channel has a multi-threaded, shallow thalweg planform (width/depth ratio: 55-125) that is mostly bar-braided with some large vegetated islands. Bed sediment is predominantly medium to fine sands within the active channel, bank, and vegetated islands, with minor pebble gravel accounting for only about 5% of the exposed bed sediment. Downstream D₅₀ values ranged between 2 mm and 0.25 mm. Prior to dam closure the surface area of vegetated islands within the active channel was 0.010 km². Since dam closure, vegetated island surface area has increased to 0.896 km². Thalweg incision has occurred throughout the study reach and ranges from over 2.5 m at the upstream end to 1.2 m downstream. The floodplain is effectively disconnected throughout the reach given the post-dam regime of reduced peak discharges.