This study identifies and characterizes hydromorphological changes along the Rapel River downstream of the first large dam built in Chile (1968). A hydromorphological analysis is carried out to assess changes on the hydrological flow regime, bed sediments, and fluvial morphology along a 19 km river reach. Results classify current global hydrological quality as “Moderate” (according to the Indicator for Hydrological Alteration in RIverS, IAHRIS), however specific indicators within this classification scheme identified quality as “Poor”. The morphological quality decreased from “Very Good” to “Good” (assessed by the Morphological Quality Index, MQI). Changes in the planform were particularly intense during the post dam period when intensive lateral mobility occurred, with the corresponding loss of secondary river branches, and with generation of straighter and regular river sections with presence of an armor layer observed along the entire river reach. Between 1991 and 2015 channel stabilization with less lateral mobility was observed, which thought to be associated with the river new equilibrium trend. River width, sinuosity and braiding index changed at different rates along the studied river reach. Our investigation demonstrates that the Rapel River experienced changes differently than those described in the literature given its lower gradient and hydraulic interaction with the Pacific Ocean.