Technical Report

Hydraulic Habitat Suitability for Rio Grande Silvery Minnow at San Acacia Restoration Sites

URL: https://webapps.usgs.gov/mrgescp/documents/Harris_2021_Hydraulic-Habitat-Suitability-for-RGSM-at-San-Acacia-Restoration-Sites.pdf

Date: 2021/09/01

Author(s): Harris A.

Publication: Prepared by U.S. Army Corps of Engineers. Prepared for U.S. Bureau of Reclamation, 100 p.

Abstract:

The Rio Grande Silvery Minnow (Hybognathus amarus, Silvery Minnow) is an endangered species that persists in the Middle Rio Grande (from Cochiti Dam tailwaters to above the Elephant Butte Dam reservoir pool near Truth or Consequences, NM). The species has been impacted by several factors: in-stream infrastructure segmenting the Middle Rio Grande; river channelization efforts which have increased active channel depths and velocities; and anthropogenic and climatic trends reducing available water in the Rio Grande basin. The Silvery Minnow has been under direct management efforts since it was listed as an endangered species in 1994. These include rescuing the Silvery Minnow from stranded pools, conducting population counts throughout the year, and augmenting the population with minnows raised in hatcheries. There have been habitat restoration efforts to countervail the channelization of the main stem of the Rio Grande in an effort to increase the extent of shallow, slow-moving, inundation areas called ‘restoration sites’. Restoration efforts are mainly configured at the edges of the channel and floodplain to generate nursery or feeding habitats for this species. There has been research regarding the spawning and movement of the Silvery Minnow to inform adaptive management actions. Scientists and others have presented recommendations to water managers to manage snowmelt captured upstream to support the Silvery Minnow.

This study characterizes hydraulic habitat on restoration sites relative to documented habitat preferences for larval and adult life stages for the Silvery Minnow. Using literature and field-measured data, a hydraulic habitat suitability curve was created and paired with 2-dimensional (2-D) hydrodynamic modeling data to map habitat areas based on velocity and depth. Field-measured hydraulics and areas of inundation at (8) habitat restoration sites in San Acacia were used for site-by-site calibration.

The hydraulic habitat effectiveness for each restoration site was estimated based on surveyed terrains and hydraulic results. The models reflect the effects of short-term sediment deposition patterns with “As-Built” (or baseline) and “Post- Runoff” (after the monsoon season) topography. In addition, adjacent floodplain hydraulics are evaluated. The evaluation compares the hydraulic habitat contribution of non-constructed, adjacent terraces to the constructed areas. The restoration sites represent different configurations of backwater, side channels, and bank line lowering on terraces that are connected to the active channel at varying degrees. The results demonstrate how project siting contributes to Silvery Minnow habitat hydraulics.

This report defines and implements site performance metrics based on the on-site hydraulics and the frequency and duration of hydrologic events. Site performance is quantified based on duration and spatial extents. Design criteria for these sites were to 1) inundate the floodplain at less than bank full conditions; and 2) increase refugia habitat for larval life stages of the minnow. However, hydraulic habitat analysis found that much of the total inundated area is too fast-moving or too deep for quality larval habitat. Large habitat sites may not fully inundate, indicating excavated area alone is not good proxy for habitat contributions.

Generally, it was found that the habitat quality degraded at lower discharges (<2000 cfs) due to sedimentation at the interface between the restoration site and the Rio Grande active channel. The sedimentation benefited higher discharges, however, likely by reducing the depths and velocities as water disperses on the site. It was also found that the post-runoff terrains had more channelized features than the designed features. This indicates that sediment transport affects the performance of these sites in various ways. The objective of this study is to present a baseline and post-runoff hydraulic analysis for restoration sites designed for Silvery Minnow habitat. The process transforms qualitative habitat characteristics (slow moving, shallow water) into spatially based quantities for performance forecasting, evaluating alternatives in adaptive management, and design analysis for future restoration sites. As with any model, field-validation is necessary. Though the hydraulic habitat preferences are available in the literature, this study recommends incremental improvement of the Silvery Minnow habitat conceptual model which can be directly analyzed via geospatial analysis.