This report on the Penobscot River drainage system, the largest and one of the most important in Maine, has been compiled chiefly from the records, reports, and maps of the United States Geological Survey and from the results of surveys made in cooperation with the Maine State Survey Commission. The report includes all data on precipitation, stream flow, water storage, and water power that were available at the end of the calendar year 1909 and is accompanied by plans and profiles of the principal rivers, lakes, and ponds in the basin (Pis. XIII-XIX, at end of volume). Stream-flow data for 1910 and 1911 will be published in Water-Supply Papers 281 and 301, respectively.