Couesius plumbeus is distinguished by having the following characters: barbel at corner of large, barely subterminal mouth; moderately pointed snout; large eye; head flattened above and below; dorsal fin origin over or slightly behind pelvic fin origin; complete lateral line, 53-70 lateral scales; 8 anal rays, and pharyngeal teeth 2,4-4,2 (Ref. 86798). Spineless fins, normal jaws, and tiny barbel at the corner of the mouth (Ref. 27547). Body moderately compressed and slender, coloration include brown to green above, dark stripe along silver gray side, darkest on young and large male, sometimes black specks on side and belly, dusky caudal spot. Large male may have red at pectoral and pelvic fin origins, corners of mouth (Ref. 86797). In some populations, breeding males develop bright orange-red patches on sides of head and at bases of pectoral fins, but the presence of this color varies from place to place (Ref. 27547).