Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
(6,237 acres) with the Arizona State Land Department (ASLD), which includes the FRSs, flood
pool areas, Powerline Floodway, and maintenance access roads. This SNA includes the project
limits and extends to the TNW via the Powerline Floodway, the East Maricopa Floodway (EMF)
and Gila River.
6.5.2. Lost Dutchman Heights
On June 29, 2010, the Corps determined that the 7,700-acre Lost Dutchman Heights (LDH)
project (Corps file No. SPL-2008-00674-SDM) and the ephemeral washes within the project area
did not have a significant nexus to the TNW (Gila River) and issued an Approved Jurisdictional
Delineation that stated there are not waters of the U.S. within the project area. That project and
the current PVR project overlap and much of the information in the LDH submittal is also
applicable to the PVR project.
6.5.3. Purpose of Significant Nexus Analysis
The purpose of the significant nexus analysis was to assess the flow characteristics and functions
of the tributaries within the project area to determine if they significantly affect the chemical,
physical, and biological integrity of the TNW. For each of the following situations, a significant
nexus exists if the tributary has more than a speculative or insubstantial effect on the chemical,
physical and/or biological integrity of the TNW.
Considerations when evaluating significant nexus include, but are not limited to the volume,
duration, and frequency of the flow of water in the tributary and its proximity to the TNW, and
the functions performed by the tributary. It is not appropriate to determine significant nexus
based solely on any specific threshold of distance (e.g. between a tributary and the TNW).
6.5.4. Conclusion of Significant Nexus Analysis
The low frequency of flow events in combination with the peak discharges volumes from the
project area ranging from approximately 80 to 150 cfs, the likelihood of flow from the project
area reaching the TNW is low. Therefore, the project waters have a low capacity to carry
pollutants or flood waters to the TNW, or to reduce the amount of pollutants or flood waters
reaching a TNW. Furthermore, the project waters have a low capacity to transfer nutrients and
organic carbon that support downstream food webs. The combined peak discharge from the
project area (244 cfs) is insignificant in comparison to peak discharge in the TNW (75,883 cfs),
and represents 0.3% of the peak flow in the TNW. Therefore, no significant nexus exists with
regard to physical and chemical integrity.
The project area provides no habitat for aquatic organisms in the downstream TNW, though
ponding areas may provide habitat for several invertebrate species. The only listed species
overlap is for the Sonoran desert tortoise which could use ephemeral washes and the adjacent
uplands as dispersal habitat. The potential for the project area to contribute biologically to the
TNW downstream is unlikely and inconsequential. Furthermore, the project waters have a low
capacity to transfer nutrients and organic carbon that support downstream food webs. Therefore,
no significant nexus exists with regard to biological integrity.
The waters analyzed do not significantly affect the chemical, physical, and biological integrity of
the downstream TNW and therefor appear not to be jurisdictional waters of the U.S. The SNA
was submitted to the Corps for concurrence in June 2012. The Corps has submitted the SNA
package to the U.S Environmental Protection Agency in January 2013. As of the date of this
report concurrence has not been received.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 51