From main simutrans config/simuconf.tab:
# now river stuff
# first river type (should be defined in pak dependent file)
# The highest number is the smallest. A river with max_speed==0 is not navigavable by ships.
#river_type[0] = river
#river_type[1] = small_river
#river_type[2] = just_the source
From pak64 pak/config/simuconf.tab
# rivers:
# The highest number is the smallest. A river with max_speed==0 is not navigavable by ships.
river_type[0] = river
river_type[1] = river2
From openpak128/config/simuconf.tab
river_type[0] = river_00
river_type[1] = river_01
river_type[2] = river_02
river_type[3] = river_03
river_type[4] = river_04
From pak128.britain/config/simuconf.tab
river_type[0] = River3
river_type[1] = River2
river_type[2] = River1
river_type[3] = River0
See? You should define it too.
And use different image for canal. It is same as for river.