In this unit students examine Old Woman Creek National Estuarine Sanctuary on Lake Erie to study the characteristics and importance of estuaries in general. Activities include the analysis of a pictured plankton sample, a transect study using computer data, a consideration of the ecological roles of various estuarine species, and a discussion of the impacts of humans and natural forces upon estuaries. Both a teacher's guide and student's manual are provided. Contained in the guide for teachers are objectives, suggested instructional approaches, an answer key, and additional information on Old Woman Creek. (Author/WB)