Argonaut-SW Monitors Culvert in Havasu National Wildlife Refuge

February, 2003 - At a site on the lower Colorado River near Needles, Arizona, the U.S. Fish and Wildlife Service (USFWS) installed an Argonaut-SW (Shallow Water) to monitor flows into the Topock Marsh. As one of the largest and most important wetlands in the region, accurate flow monitoring is needed because the marsh has fixed water rights that cannot be exceeded. The site has proved challenging to both accurately gage and monitor because it is a complex flow environment involving dam releases, long shallow channels, and the potential for reversing flow.

There are two channels that route water into the marsh from the Lower Colorado, the larger of which previously used an Argonaut-SL to monitor the input. The smaller channel, called the "Farm Ditch" (see photo*), accounts for 10-20% of the total flow and has never been monitored. The ditch is 10-20 feet (3-6 m) wide, and the water level in the channel is typically at the same level as the marsh. Additionally, the bottom is soft and heavily vegetated. As such, it is impossible to define a stage-discharge relationship or even to measure discharge accurately at any given point in time.

At one section (about a mile from the diversion structure), the channel runs into a 40-foot (12 m) long, 40-inch (1 m) diameter culvert. The culvert is always full because it is below the water level of the marsh. An Argonaut-SW was installed in the culvert through a small access pipe in the top (see photo*). Power was supplied by a small lead-acid battery and a solar panel.

Discharge data were collected for the first nine days following the installation. This data were then compared to discharge data measured from a U.S. Geological Survey (USGS) gauging station (measuring level) collected from a point about two miles upstream of the diversion into Farm Ditch.

Examining the data in Figures 1 and 2, it appears that the peak flow events at Farm Ditch typically lag the dam releases by about 24 hours. Data from Day 4 also suggests that if a major release event is inconsistent, or lasts less than 9-10 hours, there will be little or no noticeable change in flow at Farm Ditch.

• Argonaut-SW

*Photo courtesy of U.S. Fish and Wildlife Service