( ( Add ( ( ( several tributaries ) ( -LRB- ( ( small streams ) -RRB- ) ) ) ( that ( will ( ( feed ( into ( your river ) ) ) ( as ( it ( heads downstream ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB Add) (NP (NP (JJ several) (NNS tributaries)) (PRN (-LRB- -LRB-) (NP (JJ small) (NN streams)) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (VP (MD will) (VP (VB feed) (PP (IN into) (NP (PRP$ your) (NN river))) (SBAR (IN as) (S (NP (PRP it)) (VP (VBZ heads) (NP (JJ downstream))))))))))) (. .)))
Add several tributaries (small streams) that will feed into your river as it heads downstream.
(ROOT (S (NP (NNS Tributaries)) (VP (VBZ is) (NP (NP (DT the) (NN term)) (PP (IN for) (NP (JJ small) (NNS streams))))) (. .)))
Tributaries is the term for small streams.