Kostanjevica is protected as a cultural monument due to its unique location. Within the monument area is inculuded also the riverbed of the Krka River around the island. Therefore, archaeological research was planned during the restoration of city bridges conducted by the Municipality of Kostanjevica na Krki, including intensive underwater field surveys and bathymetric mapping of the riverbed.

The mentioned research procedures were carried out in the summer of 2023 under the northern and western bridges. The study aimed to determine the formation and composition of the riverbed and the distribution, quantity and composition of archaeological finds. Approximately 60 meters of the area under each bridge were examined. To achieve maximum accuracy in the collected data, the area was divided into strips composed of smaller segments measuring 2 x 2 meters before the survey. Archaeologists-divers, working in pairs, first sketched the topography of the area and then collected archaeological finds.

Subsequently, the collected objects were evaluated according to the time period in which the specific objects were made and used. The material was mostly consisting of fragments of pottery from the 18th and 19th centuries, possibly extending into the late Middle Ages and the 14th century. Older findings have not been confirmed so far, which is not surprising considering that the Krka River changed its course even after the settlement of the island in the Middle Ages. The locations of both main bridges in Kostanjevica are as old as the city itself, possibly even a bit older. The bridges were rebuilt, replaced, modernized, they collapsed, were burned, or were simply carried away by the flods over the centuries. The residents of Kostanjevica have always had the right to maintain their wooden bridges with wood from the Krakov Forest. During the inspection, remnants of at least one previous bridge construction were observed. The construction of the northern bridg (as well as the southern) can be dated between 1808 and 1824 when fortification towers of the city gate were demolished.