Regulation of River Flows and Bosque Restoration

Historically, the Middle Rio Grande (MRG) riparian area (NM’s “bosque”) was a highly dynamic habitat for a diversity of native plant and animal species. Spring snowmelts from its Rocky Mountain headwaters annually produced flows that, in most years, would push the river over its banks onto its floodplain, through the riparian community and often into land beyond. The largest floods could carry water out for a mile or more on each side of this river. For most years this overbank flooding would last for a month or more. Major monsoon storm events also could produce similar flooding but of much shorter durations. Large volumes of the waters extending over the floodplain would soak through the soils and into the shallow groundwater to irrigate the bosque plants through the year and to seep via subsurface return flows to supplement Rio Grande flows, helping to limit the potential magnitude of its channel drying during late spring and summer.

Over time, these annual floods became a problem for the businesses and homes in the growing city of Albuquerque and for other communities and agricultural areas along the river.  Consequently, an extensive series of levees and jetty-jack fields were installed to constrain the flood flows and a network of drainage ditches constructed to more quickly drain flood waters, lower the surrounding water table, and limiting soil salt accumulations to improve the area’s agricultural production. Some of these ditches, plus many others, were used to supply agricultural lands with water diverted from the river during the irrigation season. To ultimately control downstream flooding along the MRG, the US Army Corps of Engineers, our nation’s primary flood control agency, constructed Jemez Canyon Dam starting in 1953 and Cochiti Dam starting in 1965 for flood control and stormwater management during the spring and early summer.

These engineered constraints on the natural flow dynamics of the Rio Grande all produced major disruptions to historical flow energies and sediment loads transported through the river. Reaches of the river below the dams developed a scoured channel due to the discharge of “sediment hungry” water from the reservoirs. The deepening channel caused a total decoupling of the channel from its floodplain through many reaches of river. In turn, many downstream reaches experienced major aggradation of the channel from depositing of the new scoured sediment loads from upstream. With their constrained channels decoupled from the floodplain, the deposited sediment loads further elevated and disconnected the channel above its historical shallow-groundwater recharge flows.

These engineered changes all produced significant ecological effects. In turn, these changes have led to substantial regulatory restrictions on MRG water management and operations, as currently defined in the US Fish and Wildlife Service’s 2016 Biological Opinion for water operations through the MRG, which are intended to limit additional adverse effects to the federally designated threatened and endangered native species still having limited populations living in and along the river.

Historically, the native fish species in the Rio Grande became adapted to the annual cycle of riparian flooding. For example, the Rio Grande Silvery Minnow, now listed under Endangered Species Act as endangered, is the last survivor of a group of primarily small-bodied native fish species adapted to the fact that historically large spring flood flows spread their energy, not down through the channel, but horizontally over the floodplain. This effectively turned the river into a large slowly moving lake. Taking advantage of this changed environment, these fish would spawn throughout this “lake.” As documented for silvery minnow, these spring-flood spawning events led to their releasing eggs into the water column at these times when their eggs would have minimal displacement drift downstream. Relatively warmer water temperature occurring in the shallower floodplain inundation waters would tend to increase rates of embryo development and growth of hatched larval fish. Added to this, the abundance of essential food resources, including freshly wetted organic material, terrestrial invertebrates, developing algae communities, and more, were readily available to increase growth rates for developing fish, better enabling their innate abilities to swim into the channel as the inundation water recede from the floodplain.

Today, to enhance the potentials for successful spawns and recruitment of silvery minnow population into the river, water management operators are required to construct floodplain lowering, side channels, and other projects intended to allow the river channel to again connect to its floodplain, at least to the extent possible during the present day’s regulated and reduced spring flows. Also, water operations are now required to be managed spring pulse flows in such a way to promote successful spawns by this minnow. Additionally, to help ensure that young silvery minnows are available to help maintain this species in the river, an active egg collection effort is conducted each year during the spring spawns. The collected eggs are then cultured at federal, state, and city facilities. Then in November of the next year, the young fish are released back into the river to augment the resident population. Also, to minimize mortality of silvery minnows due to channel drying (called “take” under ESA), the federal, state, and city water operators are required to supplement channel flows to the extent possible using off-channel water sources to limit channel drying.

The historical high annual flood flows spread seeds of cottonwoods and other plant species across the floodplains. Some higher velocity flows would scour young floodplain plants and tumble its older trees. Also, when the floodwater receded, newly scoured avulsion channels sometime appeared. These led to the growth of new river-side willow communities and abandonment of old ones. Germination and growth of young plants on the floodplain helped to periodically renew the health of the bosque community and to generally improve its habitat quality. In ecological terms, the annual floods over floodplain and the channel avulsions produced “intermediate-disturbance, disequilibrium riparian communities” along the MRG. Today, the natural resetting and restoration bosque does not occur and often open park-like areas of shade-producing old cottonwoods canopies dominate. These over-mature cottonwoods were seeded during the large floods during the 1940s. Now, mechanical wood removal to reduce bosque fire hazards and designed revegetation efforts are commonly practiced to manage bosque health.

The Southwestern Willow Flycatcher, now listed as endangered, is characterized as a riparian-obligate bird. In the past, their nesting habitat along the MRG had been described to primarily included thickets of willow species with an overstory of scattered cottonwood. More recent descriptions characterize their breeding habitat to include both native willow and non-native plant communities, including nests in tamarisk and occasionally Russian olive. Due to these habitat changes and due to the invasion into the New Mexico of the non-native tamarisk leaf beetle, concerns about the continued stature of this species has increased. As such, goals for construction, management, and operations activities by federal, state, and city water agencies activities along the MRG, as introduced above to benefit silvery minnow population recruitment, have expanded to include goals to restore native riparian willow communities to benefit nesting success and population recruitment for this flycatcher. Dense areas of tamarisk near flycatcher nesting sites have be targeted for mechanical removal and revegetation with willow and other native plant species.