a meridional tectonic depression on Sakhalin Island, between the Western and Eastern Sakhalin mountains. The trough is 250 km long and ranges from 5 to 60 km in width. It is formed by two diverging valleys through which the Tym’ River flows to the north and the Poronai River flows to the south; the upper courses of the main rivers are separated by a poorly defined water drainage divide. The Tym’ River valley is the narrower of the two; it is covered by large taiga and contains cultivated fields on terraces. The southern section of the trough widens toward Terpeniia Gulf and is covered by the swampy Poronai Lowlands. The Tym’-Poronai Trough is the principal agricultural region of northern Sakhalin Island.