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2 THE NEBRASKA LANDSCAPE Prior to European settlement, Nebraska was a land of rivers and streams. There were few natural lakes and these were river oxbows or natural lakes in the Sand Hills. It is probable that, historically, the Sand Hills lakes had no fish or crayfish (though we really don't know). Nebraska's streams tend to flow easterly and southeasterly, all draining into the Missouri River. These streams were organized into the 13 river basins that are illustrated in the map at the beginning of this book. Nebraska is located in the center of the North American continent where climate extremes are the norm. The state is 77,355 square miles and is roughly 200 miles north to south by 400 miles east to west. The highest recorded temperature was 118ºF and the lowest was -47 ºF. The frost-free growing season ranges from 200 days in the southeast to 140 days in the west. Precipitation varies from a high of 34 inches per year in the southeast to 14 inches in the northwest corner of the state. Since 20 inches per year is considered necessary for normal crop production, about one-half of Nebraska may be considered semiarid. As a result, irrigation is prevalent which hass had some important implications for our aquatic wildlife. The Nebraska landscape has been organized into "ecological regions" or ecoregions. An ecoregion is an area that is similar in geology, soils, landforms, vegetation, climate, water resources, wildlife and human factors. Ecoregions were developed at four levels of detail. For instance, the Great Plains, extending from Canada to Mexico, is a Level I ecoregion. The Level I ecoregions were subdivided into Level II which were subdivided in Level III which were then subdivided again into Level IV ecoregions. The map below shows the Level III ecoregions which may be the most useful in describing Nebraska's landscape. It also may be of use in determining whether differences in the distributions of wildlife species (such as crayfish) might be related to the differences in the landscape. 25, 175