Middle and Lower Rhine in Roman times: a reconstruction of hydrological data based on historical sources

Even in Roman times, River Rhine had a major economical and political relevance. Although the importance of the Rhine to Romans is conversant, information on hydrological conditions is poor. To overcome this deficit, historical sources are analysed. Alongside Middle and Lower Rhine, between the city of Bingen and the German–Dutch boundary, 17 Roman settlements, camps or forts are known. Archaeological excavations of relevant buildings give information on historical water stages, discharges, hydrological extreme events or the size of riverbed. The Roman bridge over River Rhine at the city of Cologne was partly conserved through modern times. A detailed analysis of bridge construction allows the quantitative reconstruction of mean discharge. Beside Roman buildings, documentary sources are of great advantage. Several Roman historians documented geographical events like weather conditions or hydrological extreme events. After careful review of sources, writings of Tacitus are applied for a quantitative discharge reconstruction of an extreme drought in 69 AD. Tacitus describes a ship running aground due to shallow water and a battle between Roman and German troops in the riverbed. Relating to information given by Tacitus, a simplified cross section can be reconstructed. Based on the Manning equation, a possible range of discharge is estimated. Results of research show a significant lower mean discharge in the first century AD on Middle and Lower Rhine compared to recent mean discharge. In combination of a wider channel, the Romans had to deal with minor water depth, which could impede navigation. Roman times mean water stage of Lower Rhine was about two metres higher compared to modern times, which shows a significant incision of the riverbed over the last 2000 years, whereas Roman time mean water stage at Middle Rhine was lower due to minor discharge.