diff --git a/src/rashdf/plan.py b/src/rashdf/plan.py
index c886afd..f4b5149 100644
--- a/src/rashdf/plan.py
+++ b/src/rashdf/plan.py
@@ -1125,6 +1125,9 @@ def observed_timeseries_output(self, vartype: str = "Flow") -> xr.Dataset:
 
         Parameters
         ----------
+        vartype : str, optional
+            The type of observed data to retrieve. Must be either "Flow" or "Stage".
+            (default: "Flow")
 
         Returns
         -------