River: Add default reach at study creation.

src/View/Study/Window.py

@@ -89,6 +89,8 @@ class NewStudyWindow(PamhyrDialog):
 
         if self._study is None:
             study = Study.new(name, description)
+            study.river.init_default()
+
             if self.get_radio_button("radioButton_date"):
                 date = self.get_datetime_edit("dateTimeEdit_date")
                 study.use_date(date)