Hydrographic data was collected for all of Cook County. Significant flowing and standing water features that were visible on the 1998 aerial photography were stereocompiled, and include lakes and ponds as well as the sides (double lines) of flowing water features such as rivers, tributaries, canals, channels, sizable streams, etc. This data set also contains features for smaller flowing water bodies such as streams, gullies, rills, irrigation canals, intermittent streams, etc. The hydrographic polygon data layer was created to accurately depict the location and shape of flowing water features greater than five feet in width and major standing water bodies in Cook County. Elevation attributes were added to standing bodies of water (such as lakes or ponds) but were not included for double-line flowing bodies of water. It should be noted that water elevation data was collected from aerial photography flown in April 1998 and may not reflect typical or average water levels. Hydrographic attributes include feature name, tax map name, type, and elevation (for standing water bodies). The primary source of the attribute information was the Cook County Highway Department database. Hydrographic names and alias IDs are linked to lookup tables based on the hydrographic name ID attribute.