diff --git a/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.html b/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.html
index aa12a43084c645c56379cd303cc0083aea50721c..a6587c7730d3cf912b851df5f0e55f237fd3bb87 100644
--- a/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.html
+++ b/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.html
@@ -3,7 +3,7 @@
 <form>
     <mat-form-field>
         <mat-select [placeholder]="uiTextUpstreambasin" [(value)]="upstreamIndex" required>
-            <mat-option *ngFor="let b of availableIndexes" [value]="b">
+            <mat-option *ngFor="let b of availableUpstreamIndexes" [value]="b" [disabled]="! basinIsSelectable(b, false)">
                 {{ basinDescription(b, uitextRiverUpstream) }}
             </mat-option>
         </mat-select>
@@ -11,7 +11,7 @@
 
     <mat-form-field>
         <mat-select [placeholder]="uiTextDownstreambasin" [(value)]="downstreamIndex" required>
-            <mat-option *ngFor="let b of availableIndexes" [value]="b">
+            <mat-option *ngFor="let b of availableDownstreamIndexes" [value]="b" [disabled]="! basinIsSelectable(b, true)">
                 {{ basinDescription(b, uitextRiverDownstream) }}
             </mat-option>
         </mat-select>
diff --git a/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.ts b/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.ts
index f5513751b8f24e50f2bba4f9f12386f090ff6a1c..d562f03d316956b970434e34cc1c873b8981e02c 100644
--- a/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.ts
+++ b/src/app/components/dialog-new-pb-cloison/dialog-new-pb-cloison.component.ts
@@ -28,10 +28,10 @@ export class DialogNewPbCloisonComponent implements OnInit {
     ) {
         this.availableBasins = data.basins;
         this.upstreamIndex = 0;
-        this.downstreamIndex = 1;
+        this.downstreamIndex = 0;
     }
 
-    public get availableIndexes(): number[] {
+    public get availableUpstreamIndexes(): number[] {
         // add river upstream as "0"
         let ab: number[] = [ 0 ];
         for (let i = 0; i < this.availableBasins.length; i++) {
@@ -40,6 +40,32 @@ export class DialogNewPbCloisonComponent implements OnInit {
         return ab;
     }
 
+    public get availableDownstreamIndexes(): number[] {
+        let ab: number[] = [ ];
+        for (let i = 0; i < this.availableBasins.length; i++) {
+            ab.push(i + 1);
+        }
+        // add river downstream as "0"
+        ab.push(0);
+        return ab;
+    }
+
+    /**
+     * Returns true if a basin is selectable in the possible upstream basins list,
+     * considering which downstream basin is currently selected
+     * @param index index of basin
+     * @param downstream if true, inverts the test
+     */
+    public basinIsSelectable(index: number, downstream: boolean = false): boolean {
+        let ok = true;
+        if (downstream) {
+            return (this.upstreamIndex === 0 || index > this.upstreamIndex)
+        } else {
+            return (this.downstreamIndex === 0 || index < this.downstreamIndex)
+        }
+        return ok;
+    }
+
     public basinDescription(i: number, fallback: string): string {
         if (i === 0) {
             return fallback;
@@ -48,8 +74,12 @@ export class DialogNewPbCloisonComponent implements OnInit {
         }
     }
 
+    // @TODO redundant with lists filtering, useless
     public get enableValidate(): boolean {
-        return (this.upstreamIndex !== this.downstreamIndex);
+        return (
+            this.upstreamIndex !== this.downstreamIndex
+            || this.upstreamIndex === 0 // river upstream to river downstream direct connection is allowed
+        );
     }
 
     public onValidate(close = true) {