45 uploaded sonar logs; 18 feet of water level variation
Forty-five sonar logs recorded on Lake St. Croix (a wide spot on the St. Croix River in Minnesota, near Stillwater) were uploaded over the course of a year. Over that time, river levels ranged 18 feet (Figure 1). So a sonar log recorded in trip from June 2014 combined with a log from a trip in April 2015 will not produce a good map without a water-level correction. The more difficult decision facing water-resource professionals and anglers is what benchmark to use for their aggregated map.
|Figure 1.Water levels from the Army Corps of Engineers Water Control District in St. Paul MN. The USACE is one of several free online sources of real-time water-level data for dammed rivers and reservoirs.|
In the U.S., three Federal Government Agencies collect, maintain and distribute real-time water-level data for thousands of lakes, rivers and reservoirs: The U.S. Army Corps of Engineers (USACE) through its RiverGages website, the National Oceanic and Atmospheric Agency (NOAA) via the National Weather Service's National Advanced Hydrologic Prediction Service, and the U.S. Geological Survey via the National Water Information System. These databases should cover most large U.S. rivers and reservoirs.
For inland lakes, you might need to hunt a bit more for information on water levels, but Google is an amazing product and will help you determine if a state, local unit of government or water authority/district houses this information for your lake of interest. If they do, they likely publish the data on its website. In Minnesota, for example, the State Department of Natural Resources administers a popular citizen science lake-level monitoring program and I found a 10-year hydrograph for a lake I've mapped in the past (Figure 2). Note that despite some anomalies, water levels rarely vary more than 1 foot in this isolated headwater lake, which has a water-control structure at the outlet. So in this case, offsetting trips to lake elevation isn't hugely important.
|Figure 2. Water levels from Square Lake, Washington Co. Minnesota USA from citizen collected water level gauges.|
Often in rural areas, lake levels are not formally monitored and anglers and aquatic managers need different tactics to properly offset EcoSound or Insight Genesis trips. First, users could use a physical, temporary mark (crayon?) on the dock at the launch for instance to indicate the water level during each trip and offset each trip the distance between the current trip and the chosen benchmark trip. Alternatively, but perhaps less precisely, users could use shoreline indicators to understand the location of the lake's high water level and where the current elevation is relative to that high water mark.
How Offsets Affect Vegetation Maps in BioBase
EcoSound renders aquatic plant height as percent of the water column that is filled with vegetation (% biovolume). Plants that grow very close to the face of the transducer or to the surface of the water will show as red and register a 100% biovolume value (Figure 5a and b).
|Figure 5a. Percent of water depth fill with vegetation in Gibbs Lake, Rock Co., WI, USA with no depth offset. Don't use an offset to create vegetation maps in scenarios when vegetation is actually growing to the surface.|
|Figure 5b. Summary statistics from automated vegetation report|
|Figure 6a. Same map as Figure 5a but with a 1-ft depth of transducer offset. Use the transducer offset to characterize sub-surface vegetation growth just below the transducer|
|Figure 6b. Vegetation summary statistics in Figure 6a adjusted with the offset. Notice the maxium biovolume values increase with depth but always remain lower than 100% (surface of the water).|