diff --git a/namespaces/usgs/README.md b/namespaces/usgs/README.md
index 557fe593..aaeacb36 100644
--- a/namespaces/usgs/README.md
+++ b/namespaces/usgs/README.md
@@ -2,9 +2,9 @@ USGS
 ===
 
 Homepage:
-* https://waterdata.usgs.gov/
+* https://usgs.gov/
 
-These mappings are for the USGS Waterdata system. This collection will grow and evolve over time.
+These mappings are for USGS systems. This collection will grow and evolve over time.
 
 ## hydrologic-unit
 
@@ -16,6 +16,14 @@ Examples:
 * An HU06: https://geoconnex.us/usgs/hydrologic-unit/070700
 * An HU08: https://geoconnex.us/usgs/hydrologic-unit/07070005
 
+## mainstem
+
+Mainstem IDs for rivers not defined in reference rivers.
+
+Examples: 
+* https://geoconnex.us/usgs/mainstem/9085326
+* https://geoconnex.us/usgs/mainstem/7419739
+
 ## monitoring-location
 
 All monitoring locations used by the USGS Waterdata system
diff --git a/namespaces/usgs/mainstem/mainstem.csv b/namespaces/usgs/mainstem/mainstem.csv
new file mode 100644
index 00000000..f35830e8
--- /dev/null
+++ b/namespaces/usgs/mainstem/mainstem.csv
@@ -0,0 +1,2 @@
+id,target,creator,description
+https://geoconnex.us//usgs/mainstem/([0-9_]+).*$, https://hydro.nationalmap.gov/arcgis/rest/services/3DHP_all/MapServer/1/query?where=mainstemid%3D'https%3A%2F%2Fgeoconnex.us%2Fusgs%2Fmainstem%2F$1'&outFields=*&f=geojson&inSR=4269&outSR=4326, dblodgett@usgs.gov, USGS Mainstem ID