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To determine whether there are preferential relationships among individuals from the different parishes of the Ebro River delta region, we analyzed the population relationships, taking into account both the birthplaces of the spouses and their surname frequencies. We used data from the 9,085 marriages recorded in the Ebro delta area between 1939 and 1995. Using each spouse’s birthplace, we calculated the distances between the subject populations by means of the squared Euclidean distance. Also, from the surname frequencies in the marriages we obtained certain kinship measurements. In both analyses the results show a clear differentiation between the parish of Amposta and the rest of the parishes. This difference is mainly due to a greater number of marriages in which delta outsiders participated and can be related to the greater surname diversity and lesser endogamy observed in this population. On the other hand, if only endogamous marriages are taken into account, there is clearly a differentiation between the parishes from both banks of the river, with a strong homogeneity among the northside parishes. We compared the distances obtained from the birthplaces, the kinship parameters obtained from the surnames, and two geographic distance matrixes by means of a Mantel test, and the results show a strong and significant correlation between them when all marriages are considered. If, on the other hand, only endogamous marriages are taken into account, the barrier effect of the river on the interparish relationships can be appreciated.