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Gomphus adelphus is similar in appearance to its close relatives, having greenish yellow and black bodies, and a stocky abdomen with segments 7, 8, and 9 enlarged. It is distinguished from other species by black borders completely surrounding a wide yellow frons and a black inverted triangle dividing the labrum. The eyes are separated dorsally on the head. Thin yellow stripe partially divides black lateral thoracic stripe along the carina. Majority of the abdomen is black, with a pale dorsal line narrowing on the middle segments and terminating somewhere on the distal three segments. Spotted lateral abdominal lines are also narrow, but widen on the enlarged segments. Males have black claspers on terminal segment. Females generally exhibit more yellow colour on the end of the abdomen and erect spines on the head behind the ocelli. Adults are usually no more than 46mm in length (Needham et. al. 2000).

Terrestrial adults find mates near freshwater bodies during the summer months. The female will oviposit her eggs near the water where aquatic nymphs are able to enter the water once they emerge. A study on other Gomphus species reveal that June and July have the highest densities of larval emergence during the summer (Braccia et. al. 2007). As the nymphs develop, they will go through successive molts with up to 12 instars. The nymphs overwinter in diapause under the ice. After two years, the nymphs will emerge from the water and undergo the final larval molt, completing metamorphosis and becoming sexually mature winged adults.