Simplify the following text for a Grade 9 native English-speaking person.
Original: When the dispute over the ecclesiastical border between Salzburg and the Patriarchate of Aquileia broke out , Charlemagne declared the Drava to be the border .
Simplified: When the dispute over the church 's border between Salzburg and Aquileia broke out , Charlemagne declared the River Drave to be the border .