Figure 1. Bottom mapping with a Lowrance HDS-5 on Pool 2 of the Mississippi R. just downstream of the Lock and Dam on 4/27/2012.
The raw pool elevation on 4/27/2012 was 4.27 feet; still within the range of moderate drought according to the US Drought Monitor but 1.7 feet higher than the most recent low on 12/10/2011. Coincidentally, these drought levels follow historic flood levels just one year earlier (Figure 2). To demonstrate ciBioBase’s utility as a fish habitat assessment tool, we compared sizes and volumes of our mapped pool under the hydrologic conditions experienced on Pool 2 during the last year.
Figure 2. Hydrograph for the Mississippi River at St. Paul, MN (DNR ID# 20088002; USGS ID# 05331000; Data and figure courtesy of the MN DNR).
On 4/27/2012, we mapped and analyzed a 15-ft pool using the ciBioBase polygon creation tool and determined that the max depth was 17 ft, surface area was 317 m2 and the volume was 1508 m3 (Figure 3).
Figure 3. Diagnostics of a pool of interest using ciBioBase’s polygon tool.
In order to reconstruct changes to this pool under the recent low flow on December 10th 2011, we used the Z-depth Offset feature in ciBioBase to drop the elevation down 1.7 feet. In Figure 4, you can see the striking difference this reduction has on the size of this pool and consequently the amount of available fish habitat. The area on December 10th 2011 was estimated to be 3.1 m2 and volume was 9.4 m3; 100 times smaller in size and 161 times smaller in volume than on 4/27/2012. If we increase the offset by the peak flood elevation on March 30th 2011, the 15-foot hole becomes a 30-foot hole (Figure 5).
Figure 4. Polygon overlay in ciBioBase demonstrating the difference in size and volume of a 15-ft deep hole between the yearly low elevation on 12/10/2011 (pink) and during data collection on 4/27/12 (green).
Figure 5. Polygon overlay of drought elevations in 2012 (green and pink) overlain onto simulated peak flood bathymetry on 3/30/2011.
This demonstrates one potential application of ciBioBase for fish habitat studies in large rivers. We presented three striking contrasts in fish habitat conditions within one year’s time with data that took 20 minutes to collect and an hour to analyze in ciBioBase. Different hydrological scenarios can be modeled in ciBioBase and thus could be used in predictive fisheries habitat models or to reconstruct habitat conditions over some period of time.