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 tolerate standing water (in fact I often find it in and around cattail marshes in up here in Charlo). 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. Hummingbirds like this one!
 
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, nuttal's sunflower, cutleaf coneflower, western coneflower, nettle leaf giant hyssop.
 
 
 
close up photo credit: James St. John, CC BY 2.0, via Wikimedia Commons