Abstract: The pit collapse is a sloop failure that involves the interaction of water and soil. Its formation mechanism is still not fully understood because of its rapid development and difficult observation under water. A typical collapse occurring at the entrance of the left branch of Hechangzhou Island in 2012 in the lower Yangtze River, which is a reoccurring failure with a father-son’s caving pond, is chosen as a case study by comparing the topographic changes of the main channel near the mouth of the pond in the past four years before the son’s collapse. It is shown that there was siltation in the main channel with no erosion, which is inconsistent with the common situation. The topography changes of the pond upstream bend between 2011 and 2012 indicate that the pit collapse can occur when the riverbed of a shallow trough within a hydrological year is scoured. Before and after the occurrence of the son’s collapse, the bending topography of the downstream side of the father's scour pools did not change much, which indicated that the sediment generated by the pit collapse moved from the bottom to the lower level, and had little effect on the relatively high riverbed topography around it.