Wash. Rev. Code § 85.08.830
Whenever a drainage improvement district, joint drainage improvement district, or consolidated drainage improvement district within an irrigation district or irrigation districts desires to merge with an irrigation district or irrigation districts in which lands of the drainage improvement district, joint drainage improvement district, or consolidated drainage improvement district are located, it may petition the board or boards of county commissioners, as the case may be, to do so: PROVIDED, That only that portion of the drainage improvement district, joint drainage improvement district, or consolidated drainage improvement district within a particular irrigation district may merge with the irrigation district within which it is situated.
RCW 85.08.830
Merger of improvement district with irrigation district: RCW 87.03.720 through 87.03.745.