Draft for Review
The Bosque Hydrology Group
Middle Rio Grande Rehabilitation Concepts
Bernalillo to Bernardo
Introduction: The following conceptual framework for the rehabilitation of the Middle Rio Grande (MRG) is the outcome of the December 2000 Bosque Hydrology Group Meeting. The concepts presented herein focus on the reaches of the MRG between Bernalillo and Bernardo (75 river miles), since the geomorphology of the MRG is fairly consistent within these boundaries. These concepts do have applicability for the reaches north of Bernalillo and south of Bernardo, but additional concepts will need to be considered within these reaches in order to fully address reach specific issues.
All concepts presented herein are constructed to be implemented within current levee configuration of the MRG, and current water availability. Though not critical to their short-term success, these designs will be greatly enhanced by the re-introduction of greater sediment loads into the system, and the re-shaping of the spring run-off hydrograph.
Current Condition: The current condition of the MRG within the delineated 75 miles is degraded. The historical functioning of the River has been disrupted due to bank stabilization, lack of high spring run-off peak flows, sediment starvation, and invasion by non-native riparian species. The dynamics of a functioning riverine ecosystem have concurrently been disrupted.
Prior to the installation of 20th century projects designed to control the river; the river's channel was actively connected to its' flood-plain by high discharge floods; the main channel was wide (1000'-2000') and contained an active sand bed; the channel actively eroded existing flood-plain and created newer flood-plain; the riparian vegetation on the flood-plain was of a more open structure and contained areas of grasslands.
The current condition of the MRG within this reach is comprised of: a channel that is disassociated with its' former flood-plain; a 600' wide channel that has been frozen in place by jetty jacks and non-native vegetation; a great reduction in in-channel sediment; a riparian corridor that is characterized by a dense non-native under-story, extensive collection of litter. The current condition greatly favors non-native, water consuming, phreatophyte species.
Primary concerns are:
1) Channel Narrowing
Analysis of the channel width trends within the reach reveals that the planform of the river is shifting form a wide, sand bed, braided river, to a sediment starved, narrow, meandering river. The meandering planform enhances incision, further alienating the channel from the flood-plain.
2) Frozen Channel Banks
The banks of this reach have been frozen in place by jetty jack lines, non-native vegetation, and reduced annual peak discharge.
Reservoir installation, arroyo dams, bank stabilization, and non-native vegetation have all contributed to a starvation of sediment within the reach. Sediment starvation, coupled with reduced peak flows, has created the narrowing trend. It is speculated that sediment starvation may be a primary cause in the reduction of suitable silvery minnow habitat within the MRG.
4) Peak Spring Discharge
The peak spring discharge flow out of Cochiti have been, on average, greatly reduced due to the operation of the reservoir.
Biologic Consequences of Current Condition
1) Non regenerating native Cottonwood forest
Could biologists please briefly elaborate on these and add any points if need be?
2) Loss of suitable habitat for native silvery minnow
Societal Consequences of Current Condition
The fire risk within the modern bosque has been greatly increased by non native species and woody debris (dead fall).
2) Water Consumption
Water consumption within the modern bosque has been increased by non native phreatophytes.
Flooding hazards have been increased by the disassociation of the channel from the former flood-plain. This disconnect has decreaseed the potentila for flood peaks to be attenuated by the flood- plain.
Desired Future Condition: The desired future condition of the Middle Rio Grande is one where natural riverine functions have been returned to the system. These functions will allow for a dynamic, regenerating channel and an active connection between the channel and the flood-plain.
The MRG will never return to what it
once was. The Bosque Hydrology Group proposes rehabilitation efforts to work
within the scaled down river framework. The concept of a scaled down "Mini Rio
Grande" was arrived at during discussions of the fall 2000 meeting. It is the
consensus of BHG members that the MRG, within the Albuquerque Reach, can be
rehabilitated within the "Mini Rio Grande Framework". All aspects of the Middle
Rio Grande have been reduced in size including: flood-plain width, channel
width, peak discharge, sediment load. Working within the scaled down framework,
a new functioning system can be achieved through rehabilitation efforts that
will return functioning to the system. Historic widths, sediment loads and peak
flows are not needed to be return functioning to the system. The functioning of
the system can be optimized while working within the scaled down Rio Grande or,
if you will "Rito Grande".
The following rehabilitation conceptual designs address returning system function to the MRG. These concepts are the result of a consensus process amongst BHG members and are based on existing test restoration projects and baseline information about the historic and modern functioning of the MRG.
Rehabilitation Conceptual Designs for un "Rito Grande" from Bernalillo to Bernardo:
The following restoration concepts
are in alphabetical order and do not have any importance weight placed on them.
Destabilize banks of the river in conjuction with bank lowering. Remove jetty jack lines that are frozen within the banks, Remove non native vegetation and associated root structures along the bank.
Using the Albuquerque Over-bank Project as a successful test, implement bank lowering at a rate of 500 acres per year for the next 20 (?) years. Return sediment removed from banks into the river.
Channels in Terrace
Cut channels within the terrace to promote a better connection between the channel and the flood-plain. These channels can be cut at different levels to allow for varying connectivity (ie, some could wet at 5000 cfs, some 2000 cfs and others at base flow. This will promote the natural recruitment of native vegetation.
Grade Control Facility
Place Grade Control Facilities (as done at Santa Ana Pueblo) at key locations to promote upstream overbanking and dissuade further incision of the channel.
Increase Spring Discharge
Increase the spring discharge peak from Cochiti Reservoir. This will promote a greater connectivity between the main channel and the flood-plain.
Seek out sources of sediment to be returned to the system. Possible sources include: 1) opening arroyo mouths, 2) Bank Lowering, 3) Jemez Reservoir, 4) pass through in Cochiti Reservoir, 5) Galisteo Reservoir, 6) Re-outfitting MRG Conservancy District's diversion structure to divert less sediment.
Understory Exotic Removal
Remove exotic understory from the bosque. Remove dead fall and litter. This will reduce water consumption, reduce fire potential, improve flood-plain ability to attenuate large floods, and improve forest health.
Wetland/ pond construction
Constructed on the flood-plain. Will make great fly catcher habitat, and, when the river migrates into a wetaland, backwater habitat for minnows.
Woody Debris in River
Place woody debris from thinned bosques into the river. This will allow for greater braiding and return organic materials to the modern channel.