diff --git a/hydromt/data_adapter/dataframe.py b/hydromt/data_adapter/dataframe.py
index ab1b8a8d7..67e5691cc 100644
--- a/hydromt/data_adapter/dataframe.py
+++ b/hydromt/data_adapter/dataframe.py
@@ -167,6 +167,7 @@ def to_file(
             driver = "csv"
             fn_out = join(data_root, f"{data_name}.csv")
             obj.to_csv(fn_out, **kwargs)
+            kwargs["index_col"] = obj.index.name
         elif driver == "excel":
             fn_out = join(data_root, f"{data_name}.xlsx")
             obj.to_excel(fn_out, **kwargs)