The word tributaries is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 15
Highest Scoring Words With Friends™ Play In The Letters tributaries
:
Seven Letter Word Alert: (45 words)
abutter, airiest, arbiter, artiest, artsier, astuter, attirer, attires, baiters, barites, barters, batters, battier, bitrate, bitsier, bitters, bittier, braiser, brasier, bruiser, bursate, bustier, butters, butties, iratest, irisate, rarebit, ratbite, ratters, rattier, rebaits, reburst, restart, rustier, ruttier, situate, starter, stature, striate, tarries, tastier, terabit, tribute, truster, turrets