Opportunistic observations of Nearctic River Otter (Lontra canadensis) were collected by volunteer observers in Humboldt and Del Norte counties to describe group and litter size patterns. In the first 5 y, 709 records were sent by email, mail, or an online form. River Otter pups were characterized relative to adult body size. Group sizes, ranging from 1 to 9 otters, were larger in summer and smaller in winter. Litter size ranged from 1 to 4 pups and declined from an average of 3.2 in spring to 1.6 in fall. Observers recorded 49 litters among 23 watercourses, with otters from some sites producing pups in multiple years. This Citizen Science approach to monitoring can provide valuable information to assess basic population parameters that might otherwise be unobtainable.