X0101 EFFECT 33 LIVE 2010-09-22 0000950123-10-086665 485APOS 0001352280 RIVERSOURCE SERIES TRUST 333-131683 S000030091 Columbia Marsico Flexible Capital Fund C000092420 Columbia Marsico Flexible Capital Fund Class A S000030091 Columbia Marsico Flexible Capital Fund C000092421 Columbia Marsico Flexible Capital Fund Class C S000030091 Columbia Marsico Flexible Capital Fund C000092422 Columbia Marsico Flexible Capital Fund Class I S000030091 Columbia Marsico Flexible Capital Fund C000092423 Columbia Marsico Flexible Capital Fund Class R S000030091 Columbia Marsico Flexible Capital Fund C000092424 Columbia Marsico Flexible Capital Fund Class Z