diff --git a/modules/ui/rivers-creator.js b/modules/ui/rivers-creator.js
index 22f88b70..0a2d62c6 100644
--- a/modules/ui/rivers-creator.js
+++ b/modules/ui/rivers-creator.js
@@ -79,7 +79,7 @@ function createRiver() {
     const riverCells = createRiver.cells;
     if (riverCells.length < 2) return tip("Add at least 2 cells", false, "error");
 
-    const riverId = last(rivers).i + 1;
+    const riverId = rivers.length ? last(rivers).i + 1 : 1;
     const parent = cells.r[last(riverCells)] || riverId;
 
     riverCells.forEach(cell => {
diff --git a/modules/ui/tools.js b/modules/ui/tools.js
index 5243cd95..8783fded 100644
--- a/modules/ui/tools.js
+++ b/modules/ui/tools.js
@@ -539,7 +539,7 @@ function addRiverOnClick() {
 
   const {alterHeights, resolveDepressions, addMeandering, getRiverPath, getBasin, getName, getType, getWidth, getOffset, getApproximateLength} = Rivers;
   const riverCells = [];
-  let riverId = last(rivers).i + 1;
+  let riverId = rivers.length ? last(rivers).i + 1 : 1;
   let parent = riverId;
 
   const initialFlux = grid.cells.prec[cells.g[i]];