In February of 2003, a SonTek/YSI Argonaut-SW (Shallow Water) was installed at a site on the lower Colorado River near Needles, Ariz. by the U.S. Fish and Wildlife Service.
This Acoustic Doppler instrument was installed to monitor flows into the Topock Marsh, a particularly difficult site for flow measurement.
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 gauge 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 another of SonTek/YSI’s Acoustic Doppler instruments to monitor the input.
The smaller channel, called the "Farm Ditch,” accounts for about 10 to 20% of the total flow and has never been monitored. The ditch is 10 to 20 ft wide and the water level in the channel is typically at the same level as the marsh.
Additionally, the bottom of the channel is soft and heavily vegetated which makes it impossible to define a stage-discharge relationship or even to measure flow accurately at any given point in time.
At one section (about a mile from the diversion structure), the channel runs into a 40-ft long, 40-in. 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. Power was supplied to the instrument by a small lead-acid battery and a solar panel.
Flow data were collected for the first nine days following the installation. Data were then compared to discharge data measured from a U.S. Geological Survey gauging station. This station is located about two miles upstream of the diversion into Farm Ditch and measures the water level to determine the total flow.
Examining the collected data, peak flow events at Farm Ditch typically lag the dam releases by about 24 hours. Data collected during the fourth day also suggest that if a major release event is inconsistent, or lasts less than 9 to 10 hours, there will be little or no noticeable change in flow at the Farm Ditch site.
The success of this monitoring study indicates the versatility of the Argonaut-SW under challenging conditions and the informative data that can be gathered through its use.
Please contact SonTek/YSI at +1 858 546 8327 or visit www.sontek.com  for more information on how the Argonaut-SW (or any of our other flow and current measurement instruments) can make tackling your flow measurement requirements easier.