Tracking Change in Land Cover Within Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River Between 1975 and 1994 1,2 Shawn Weick 1Saint Mary’s University Department of Resource Analysis of Minnesota, 700 Terrace Heights, Winona MN 55987, USA; 2 U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 575 Lester Ave, Onalaska, WI, USA Keywords: Geographic Information Systems, Mississippi River, Lake Onalaska, Navigation Pool 7, Land Cover Change, Emergent Vegetation Abstract Percent land cover as well as land cover type were compared using automated coverages of Lake Onalaska for the years 1975, 1989, and 1994. Lake Onalaska is a shallow (mean depth = 1.3m), 2,800 ha impounded lake, which is a vital part of Navigation Pool 7 of the Upper Mississippi River. Changes were observed using the modified thirteen-class land cover/land use coverages by performing statistical summaries and overlay analyses. Overall, between 1975 and 1994 within Lake Onalaska, there were declines of 481 hectares (11.5%) of submergent and rooted floating leaf vegetation types. Open water habitat increased by 397.8 hectares (9.5%). Emergent vegetation increased 30.6 hectares (6.2%) from 1975-1989 and decreased 13.4 hectares (2.6%) between 1989 and 1994 for a total gain of 17.2 hectares (3.5%). Areas of Sagittaria (arrowhead) declined by 14.0 hectares (4.1%) in the period between 1975 and 1989. There was an increase of Sagittaria between 1989 and 1994 of 62.4 hectares (18.9%) with a net overall increase of 48.4 hectares (14.1%). Introduction Distribution and abundance of these changes can better inform river vegetation within the Upper Mississippi managers so they can make better River System (UMRS) has changed decisions in the future. dramatically since the inundation of the In past years, Lake Onalaska, in river in the 1930s. These changes, Pool 7 of the UMRS, was known as a because of the construction of Locks and heavily vegetated and nutrient rich Dams, have been depicted as elevated impounded lake (Green, 1960). It was river stages (Grubaugh and Anderson, an area known for its fish and wildlife 1988), increased surface area (Chen and populations. It is believed that this area Simons, 1986), and higher sedimentation has become less productive and the rates behind dams (Bhowmik and amount of submersed and emergent Adams, 1986). Because of the rivers vegetation has been diminishing. There importance as an economic and are many possible assumptions that can recreational resource it is important to be made to explain such change in have a better understanding of how these vegetation within the landscape, such as changes have affected its landscape and successional changes, increased surrounding floodplain. Understanding sedimentation rates, change in 1 hydrograph, the influence of pesticides states (Minnesota, Wisconsin, Iowa, and herbicides from agricultural runoff, Illinois, Missouri). and/or the lack of useable nutrients As part of the UMRS, Pool 7 is within the system. approximately 11.8 river miles long. It In an effort to characterize the extends from Lock and Dam 6, at magnitude of such change in Lake Trempealeau, Wisconsin to Lock and Onalaska, this project was developed to Dam 7, at Dresbach, Minnesota. Lake analyze digital land cover/land use Onalaska, a vital part of Pool 7, lies coverages of the area over a period of between the Black River Delta and the time using a geographic information spillway of Lock and Dam 7. The lake system (GIS). In this way trends can be is shallow (mean depth = 1.3 m) and explored and theories can be generated encompasses 2,800 hectares of open to help explain changes that have water, islands, and channel ways occurred. (Figure 1). The objectives of this study were to: • characterize the change in the amount and type of land cover within the area of Lake Onalaska between 1975 and 1994 • understand the past trends to assist managers in the future • explore trends and generate theories In recent years, there have been many monitoring efforts as well as a number of research projects that have focused on the Lake Onalaska area. These efforts have attempted to document some of the trends that have taken place and explain the driving forces behind such changes. Methods The Mississippi River, in its entirety, is Figure 1. Location of Pool 7 and the Study Area comprised of 2,320 miles of waterway that extends from Lake Itasca, Lake Onalaska was known for its Minnesota to the Gulf of Mexico. It is a aquatic vegetation since it was formed great system that is home to more than by impoundment in 1937 (Green, 1960). 154 species of freshwater fish and over The study area that was chosen for this 50 freshwater mussels. The river also analysis consisted of all of Lake acts as a migration corridor for Onalaska and areas just up river. The thousands of bird species. The UMRS west study area boundary follows the refers to the upper 1,300 miles of main channel from lock and dam 7 to waterway that forms the borders of five river mile 709.0. The upper study area 2 boundary follows the Black River rendered utilizing rasterized coverages System from the main channel back to of the study area that were created in the where it enters the Mississippi River Grid module of Arc/Info. Employing floodplain. The east boundary is the the rasterized coverages, overlay shoreline that borders the city of analyses were completed to reveal what Onalaska, Wisconsin and the lower changes occurred over the nineteen-year boundary is the spillway and shoreline period. The operation looked at the type around the LaCrosse Airport. of land cover that existed for one year The following analyses were and compared it to that present in a later completed using a Geographic year. This was done by looking at the Information System (GIS) and same exact cells (cell size of five meters) Environmental Systems Research within the two coverages to see if they Institute's (ESRI), Arc/Info software. were classified as the same land cover The data that were utilized for this class. If the two cells being compared project consisted of Arc/Info land differed in classification, then the change cover/land use coverages of pool 7 for was noted. This comparison of cells was the years 1975, 1989, and 1994. These done for the entire coverage and for the coverages were transferred from the data periods 1975 to 1989, 1989 to 1994, and library of the U. S. Geological, Upper the entire period 1975 to 1994. Midwest Environmental Sciences There were two steps that had to (UMESC) in Onalaska WI. be taken to perform this analysis. First, The Great River Environmental the coverages were converted into Action Team I (GREAT I) automated rasterized format, and secondly a the 1975 coverage from 1:9,600-scale condition statement was used focusing color infrared photography. The on one land cover class to make the interpreted data were transferred to comparison between the coverages. For 1:24,000-scale United States Geological instance, when open water was analyzed, Survey (USGS) quadrangle maps and the condition statement looked at areas manually digitized. The coverage was of open water in 1975 and identified then converted to an Arc/Info coverage how those areas existed in 1989. An by the EMTC. area report that was generated showed The 1989 and 1994 coverages the amount of area that remained were automated by the EMTC under the unchanged over time. It also showed the Long Term Resource Monitoring amount of area and the vegetation type Program (LTRMP) from 1:15,000-scale that the area changed into, if indeed a color infrared photography. Photo change did occur. interpretation was performed with a This analysis showed how much stereoscope. The interpreted data were of the area remained unchanged and how transferred to mylar overlays, registered much of the area changed into a different to a USGS 7.5’ quadrangle map, and land cover class. This gives a better manually digitized into a GIS as spatial representation of the change that Arc/Info coverages. has taken place within the landscape. Summary statistics, such as total From this, theories were developed to try hectares of land cover and percent land to pin point the driving mechanisms cover were compared using polygon causing the change in plant presence coverages. Further analyses were within Lake Onalaska. 3 Results grasses/forbs, with a considerable reduction in submergent vegetation, General Trends among Classes submergent rooted floating vegetation, and rooted floating vegetation. Land cover analysis for the three years 1975, 1989, and 1994 were done to help understand how the Lake Onalaska area has changed. An eleven-class vegetation classification was used to classify land cover/land use types. It utilized the existing original thirteen-class classification, but modified it by combining three classes that were comprised of submergent-rooted floating-emergent, rooted floating- emergent, and emergent vegetation. These were grouped into a class of rooted floating/emergent vegetation. This modification was done to help alleviate discrepancies that occurred because of a difference in interpretation as the coverages were created by different organizations with different Figure 2. Area of Change 1975, 1989, 1994 interpreters. An example of the type of To help understand the changes in the change is shown graphically in figure 2. areas of vegetation over the nineteen- Some of the changes that took place year period, the changes are divided into (Table 1 and Figure 3) included an three periods, the first from 1975 to increase in open water and areas of 1989, the second from 1989 to 1994, and the third from 1975 to 1994.