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river system[′riv·ər ‚sis·təm]
a group of rivers discharging water by way of a common flow or system of channels into a sea or lake. A river system consists of the main river (the trunk of the system) and primary, secondary, and later-order tributaries. Rivers that flow directly into the main river are called primary tributaries, tributaries of primary tributaries are called secondary tributaries, and so forth. The Oskol River, which flows into the Sever-skii Donets River, is a primary tributary, and the tributaries of the Oskol are secondary tributaries. Large river systems include up to 20 orders of tributaries. A river system is named after the main river, which is usually the longest and has the greatest discharge in the system.