성덕리는 완주군 봉동읍 낙평리와 함께 고산천의 자유 곡류에 의해 형성된 범람원이 발달한 평야지대다.
Seongdeok-ri, along with Nakpyeong-ri, Bongdong-eup, Wanju-gun, is a plain area where a floodplain formed by free meander in Gosancheon was developed.