diff --git a/server/src/atlas/gen_queue.rs b/server/src/atlas/gen_queue.rs
index 9c0d3f966..0f27e1928 100644
--- a/server/src/atlas/gen_queue.rs
+++ b/server/src/atlas/gen_queue.rs
@@ -1183,10 +1183,12 @@ fn run_work_item(
                 // on body_id alone, so a step-canvas request and a legacy
                 // district_window request for the same body share one
                 // ~45 ms run_layer1 re-derive, never pay it twice.
+                // body_params threads the real moisture ceiling into the
+                // T-1184 hydrology solve (merge seam resolution).
                 let (l1, ta) = terrain_cache
                     .lock()
                     .unwrap()
-                    .get_or_derive(body_id, &working);
+                    .get_or_derive(body_id, &working, Some(body_params.as_ref()));
                 let climate = ClimateConstants::default();
                 let raw = crate::atlas::step_canvas::build_step_canvas(
                     *body_seed,