This species is unmistakable in the northern hemisphere due to its large spatulate bill. The breeding drake has an iridescent dark green head, white breast and chestnut belly and flanks. In flight, pale blue forewing feathers are revealed, separated from the green speculum by a white border. In early fall the male will have a white crescent on each side of the face. In non-breeding (eclipse) plumage, the drake resembles the female.
The female is a drab mottled brown like other dabblers, with plumage much like a female mallard, but easily distinguished by the long broad bill, which is gray tinged with orange on cutting edge and lower mandible. The female's forewing is gray.
They are 48 cm (19 in) long and have a wingspan of 76 cm (30 in) with a weight of 600 g (1.3 lb).