DOWNSTREAM EFFECTS OF COCHITI DAM ON THE MIDDLE RIO GRANDE, NEAR ALBUQUERQUE, NEW MEXICO

Dams, levees, and bank stabilization projects over the 20th century have confined the middle Rio Grande to a narrow floodplain and altered discharge, sediment supply, and vegetation.  In the 25 km study reach between Bernalillo and Albuquerque, a dense riparian forest of cottonwood and exotic saltcedar and Russian olive has replaced saltgrass and patchy cottonwood stands. Formerly, bed sediment was predominantly sand, with gravel more common in the upper reach and at high flows. The overall postglacial trend has been aggradation, and until the late 1940s, aggradation within the levees posed a major problem. Cochiti dam (47 km above Bernalillo, closed in 1973) and other impoundments now trap an estimated 80% of sediment inflow to the study reach.  This induced a downstream-migrating wave of incision and bed coarsening, initially at ~5 km/yr.  Migration has slowed, and a transition zone now lies within the study reach between a relatively low width-depth ratio, dominantly single-thread to island-braided, mostly gravel-bedded channel, and a broader, shallower sand-bedded bar-and-island braided channel. A post-1942 gage record shows that mean peak discharge remained similar after Cochiti dam regulation, but infrequent flows > 283 m³/s have been eliminated. Our goals are to explain the multiple causes of recent channel change and determine if the transition zone continues to migrate downstream.

Months after closure of Cochiti, bed sediment in the study reach coarsened from an average of fine sand (D₅₀ = 0.18 mm) to medium sand (D₅₀ = 0.35 mm).  Currently, bed sediment fines downstream from D₅₀ of 64-16 mm to 0.70-0.35 mm. Incision of up to 2.5 m has occurred in the upstream reach.  Stabilization structures have prevented lateral channel change along most of the reach and also promoted incision. The area of stable, vegetated islands increased from 9,930 m² in 1972 (2 islands) to 895,520 m² over 144 islands in 2001; saltcedar and Russian olive replace willows as island surfaces are abandoned by incision. Water surface slope in the upper study reach is near-constant at 0.0008 for Q = 35 m³/s.  Minor slope decreases above large arroyo mouths and field observations indicate that coarse sediment inputs remain near these junctions, but their current sand contributions and the overall sediment budget in the study reach remain to be elucidated.