 Most of the oblast is in the Amur's drainage basin, although the rivers in the northwest drain into the Lena and the rivers in the northeast drain into the Uda.The longest rivers include the Amur, Bureya, Gilyuy, Nyukzha, Olyokma, Selemdzha, and Zeya.The Zeya begins in the mountains in the northeast, and its middle reaches are dammed to create the huge Zeya Reservoir, which sprawls over .