North Simpson Farm Mitigation Project: Conceptual Restoration Plan

North Simpson Farm Mitigation Project: Conceptual Restoration Plan

Oxbow Ecological Engineering (OEE) and Natural Channel Design (NCD) collaborated with representatives from Tucson Audubon Society (TAS) in the development of a conceptual restoration plan for the North Simpson Farm parcel, a 640 acre site near Tucson that is split by the Santa Cruz River. As part of the work, OEE and NCD toured the site and discussed and identified potential issues, constraints, and specific restoration objectives related to the planned mitigation project for the area. This site reconnaissance also gave OEE and NCD a chance to get a read of the landscape and ground-truth pre-planning site interpretations of the hydrology and landforms.

Based on information gleaned from the site visit, review of existing reports and assessments provided by TAS, and provisional site assessments based on historical and current aerial photography and LIDAR topographic information, OEE and NCD developed a DRAFT Conceptual Improvement Plan and DRAFT Feasibility-Level Cost Estimate.  The conceptual plan addressed the goals and objectives previously outlined by TAS (Adapted from the Tucson Audubon Society In-Lieu Fee Mitigation Program Concept Plan: Lower Santa Cruz River Project June 26th, 2015):

  • Restore overbank flows and floodplain function by removing large earthen dikes directly adjacent to the river corridor. Construct a new earthen dike on the south-side of the river, offset from the corridor, to maintain the existing flood control capacity of the current dike configuration.
  • Reconnect historic floodplain channels and construct additional backwater “potholes” to help distribute, attenuate, and harvest storm-flows allowing for increased riparian vegetation and water infiltration.
  • Enhance capacity of the newly accessible floodplain to attenuate flows and store moisture by plowing historically compacted soils and replanting with native grass.
  • Facilitate the transition of the riparian vegetation community from cottonwood-willow to mesquite-hackberry woodland. For nearly a decade, the Lower Santa Cruz River maintained a healthy cottonwood/willow gallery forest through the North Simpson Farm and provided key habitat connectivity both along the river itself (the Santa Cruz River Riparian Movement Area) and east-west from the Tortolita and Tucson Mountain Ranges, including Saguaro National Park West, to what is now the Ironwood Forest National Monument, within Pima County. These wildlife linkages also extend north, into Pinal County. The Lower Santa Cruz River cottonwood-willow gallery forest was maintained by effluent flows. With the cessation of these perennial surface flows in April 2014, the channel’s subsurface water level is expected to quickly drop to the surrounding water table level (~150′), with drastic effects on channel vegetation. Without this Project, the hydro-riparian cottonwood-willow gallery forest in this reach will convert to predominantly a deciduous salt cedar thicket interspersed with Johnsongrass, blue panic grass, buffelgrass, and other invasive species. The Project will intervene by managing and facilitating the transition of this reach to the vegetation community most appropriate for intermittent flooding and a lowered water table, specifically riparian mesquite-hackberry woodland. Establishing these appropriate natives while the remnants of the cottonwood-willow gallery exist will assist in maintaining functional habitat connectivity along this riverine wildlife corridor.

Location:

Santa Cruz River, Tucson, Arizona

Client:

Tucson Audubon Society

Design Team:

Oxbow Ecological Engineering, LLC

Natural Channel Design, Inc.