diff --git a/2018/day17/src/main.rs b/2018/day17/src/main.rs
index 242356f..de461d5 100644
--- a/2018/day17/src/main.rs
+++ b/2018/day17/src/main.rs
@@ -166,6 +166,48 @@ fn pour_over(m: &mut Map, start_x: usize, start_y: usize) {
     }
 }
 
+fn lock_water(m: &mut Map, bounds: &MapBounds) {
+    for y in bounds.min_y..=bounds.max_y {
+        for x in 0..MAP_SIZE {
+            if m[x][y] == FALLING_WATER {
+                let mut locked = true;
+                for k in (0..x).rev() {
+                    if m[k][y] == WALL_SPACE {
+                        break;
+                    } else if m[k][y] == EMPTY_SPACE {
+                        locked = false;
+                        break;
+                    }
+                }
+                for k in (x+1)..MAP_SIZE {
+                    if m[k][y] == WALL_SPACE {
+                        break;
+                    } else if m[k][y] == EMPTY_SPACE {
+                        locked = false;
+                        break;
+                    }
+                }
+                if locked {
+                    m[x][y] = WATER;
+                } else {
+                    for k in (0..x).rev() {
+                        if m[k][y] == EMPTY_SPACE || m[k][y] == WALL_SPACE {
+                            break;
+                        }
+                        m[k][y] = FALLING_WATER;
+                    }
+                    for k in (x+1)..MAP_SIZE {
+                        if m[k][y] == EMPTY_SPACE || m[k][y] == WALL_SPACE {
+                            break;
+                        }
+                        m[k][y] = FALLING_WATER;
+                    }
+                }
+            }
+        }
+    }
+}
+
 fn count_water(m: &Map, bounds: &MapBounds) -> usize {
     let mut squares: usize = 0;
     for y in bounds.min_y..=bounds.max_y {
@@ -198,9 +240,17 @@ fn count_resting_water(m: &Map, bounds: &MapBounds) -> usize {
 fn main() {
     let (mut m, bounds) = build_map_from_file("input");
     pour_over(&mut m, 500, 0);
-    print_map(&m, 0, MAP_SIZE - 1, 0, MAP_SIZE - 1);
+    if cfg!(debug_assertions) {
+        print_map(&m, 0, MAP_SIZE - 1, 0, MAP_SIZE - 1);
+    }
     let water_squares = count_water(&m, &bounds);
     println!("found {} water squares", water_squares);
+    lock_water(&mut m, &bounds);
+    if cfg!(debug_assertions) {
+        print_map(&m, 0, MAP_SIZE - 1, 0, MAP_SIZE - 1);
+    }
+    let water_squares = count_water(&m, &bounds);
+    println!("found {} locked water squares", water_squares);
     let resting_squares = count_resting_water(&m, &bounds);
     println!("found {} resting water squares", resting_squares);
 }