Combine rhubarb, water, and powdered pectin in a flat-bottomed kettle; stir well. Bring to a boil, stirring frequently. Add sugar, and return to a rolling boil. Boil 1 minute, stirring frequently. Remove from heat. Stir 7 minutes; skim off foam with a metal spoon.
Quickly ladle marmalade into hot sterilized jars, leaving 1/4- inch headspace. Cover at once with metal lids, and screw bands tight. Process marmalade in boiling-water bath 10 minutes.