diff --git a/dataretrieval/nwis.py b/dataretrieval/nwis.py
index 57cbccf6..0f3a1a99 100644
--- a/dataretrieval/nwis.py
+++ b/dataretrieval/nwis.py
@@ -31,7 +31,7 @@
 WATERDATA_SERVICES = ['qwdata', 'measurements', 'peaks', 'pmcodes', 'water_use', 'ratings']
 
 
-def format_response(df, service=None, **kwargs):
+def format_response(df: pd.DataFrame, service=None, **kwargs) -> pd.DataFrame:
     """Setup index for response from query.
 
     This function formats the response from the NWIS web services, in
@@ -79,7 +79,7 @@ def format_response(df, service=None, **kwargs):
     return df.sort_index()
 
 
-def preformat_peaks_response(df):
+def preformat_peaks_response(df: pd.DataFrame):
     """Datetime formatting for the 'peaks' service response.
 
     Function to format the datetime column of the 'peaks' service response.