I feel like there are lots of tall yellow flowers that like moist soils, and here, finally, is the perfect pairing: a tall purple plant! Each flowering stalk can be around 6 ft tall, with a whole foot of that dedicated to little bright purple flowers. 
 
Blue vervain is a tall prolific bloomer that enjoys easy living in irrigated or riparian areas. It can supposedly tolerate short periods of standing water, but I haven't tested that. It's pest-resistant, a pollinator favorite (from my observations), and is beautiful. It's especially showy planted in patches since the flowers themselves are quite small.
 
If you have an irrigation ditch, or an irrigated fenceline or border that needs something with height, consider my 'dream team' of tall perennials: blue vervain, maximillian sunflower, cutleaf coneflower, western coneflower, nettle leaf giant hyssop.
 
 
 
close up photo credit: James St. John, CC BY 2.0, via Wikimedia Commons