Impact of agricultural runoff on a shallow fluvial lake and wetland in the Mincio River basin (Italy)

Fluvial lakes, due to their short renewal time, are particularly affected by upstream processes. This study focuses on the morphological evolution of the Lake Superiore (Mantua, Northern Italy) and of the upstream wetland. The lake is a shallow fluvial reservoir, regulated by a dam on the Mincio River, which is the main outlet of Lake Garda. The case study is located in the central part of the Po River Valley, one of the most productive agricultural areas in Italy. The farmland is irrigated by a huge ditch network with a total length of approximately 2000 km, almost 30 times the length of the Mincio River itself (75 km). Many of the river’s main tributaries are agricultural canals, resulting in a considerable amount of nutrients and fine sediment inputs due to surface runoff. During high rainfall events, the sediment plume released from these channels is clearly visible also from satellite images and can often reach the lake. As a result, Lake Superiore is hypertrophic due to nutrient loads and water stagnation, with a Secchi depth lower than 1 m and dissolved oxygen saturations that may exceed 400% during summers.

Using the previously tested “Deeper CHIRP+” low-cost SONAR, between 2023 and 2024 we acquired a detailed bathymetric map of the lake and of the wetland’s main channel network. We compared these maps with formerly acquired bathymetries of the two areas dating back to 2006 and 2016 respectively. Both areas showed a clear trend of deposition, with maximum differences in bed elevation of the order of 1 m, approximately 1/3 of the mean depth of the lake. The wetland, which has high naturalistic value (included in Nature 2000 sites), resulted particularly threatened by landfilling, with a risk of channels closure and loss of aquatic wildlife habitat.

Since February 2024, we have been acquiring monthly measurements of discharge, water quality, and suspended solids concentration in the Mincio River and in two of its main tributaries: the Goldone and Osone canals. Osone showed the highest values of suspended sediments, with a TSS concentration of 140 mg/L at a discharge of 12 m³/s, resulting in a load of 145 tons/day entering the wetland, while the incoming discharge in the Mincio River was comparable. Regarding nutrients, it reached a maximum concentration of 16 mg/L of total nitrogen, further worsening the ecological state of the system.

Due to either water scarcity or flood protection issues, management authorities of the Mincio River are skeptical about lake flushing activities. However, it could be possible to plan effective flushing operations within an integrated management framework. Increasing the discharge from the dam immediately after heavy rainfall events may reduce sedimentation at the time of maximum inputs. To achieve this objective, a good knowledge of the water and sediment distribution in the channel network is needed.  For this reason, we are developing a numerical model to predict sedimentation rates in the wetland for different through-flowing discharges.