diff --git a/CMakeLists.txt b/CMakeLists.txt
index ea08aa3..2643c81 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,7 +9,7 @@ include(FetchContent)
 FetchContent_Declare(
   topotoolbox
   GIT_REPOSITORY https://github.com/TopoToolbox/libtopotoolbox.git
-  GIT_TAG 2024-W40
+  GIT_TAG main
 )
 FetchContent_MakeAvailable(topotoolbox)
 
diff --git a/src/lib/grid.cpp b/src/lib/grid.cpp
index 5bb92cf..79c7843 100755
--- a/src/lib/grid.cpp
+++ b/src/lib/grid.cpp
@@ -19,15 +19,17 @@ namespace py = pybind11;
 //   dem: A NumPy array representing the digital elevation model.
 //   dims: A tuple containing the number of rows and columns.
 
-void wrap_fillsinks(py::array_t<float> output, py::array_t<float> dem, 
+void wrap_fillsinks(py::array_t<float> output, py::array_t<float> dem,
+                    py::array_t<uint8_t> bc,
                     std::tuple<ptrdiff_t, ptrdiff_t> dims){
 
     float *output_ptr = output.mutable_data();
     float *dem_ptr = dem.mutable_data();
+    uint8_t *bc_ptr = bc.mutable_data();
 
     std::array<ptrdiff_t, 2> dims_array = {std::get<0>(dims), std::get<1>(dims)};
     ptrdiff_t *dims_ptr = dims_array.data();
-    fillsinks(output_ptr, dem_ptr, dims_ptr);
+    fillsinks(output_ptr, dem_ptr, bc_ptr, dims_ptr);
 }
 
 // wrap_identifyflats: 
diff --git a/src/topotoolbox/flow_object.py b/src/topotoolbox/flow_object.py
index 07b56e6..3bf218a 100644
--- a/src/topotoolbox/flow_object.py
+++ b/src/topotoolbox/flow_object.py
@@ -15,7 +15,8 @@ class FlowObject():
     digital elevation model (DEM).
     """
 
-    def __init__(self, grid: GridObject):
+    def __init__(self, grid: GridObject,
+                 bc: np.ndarray | GridObject | None = None):
         """The constructor for the FlowObject. Takes a GridObject as input,
         computes flow direction information and saves them as an FlowObject.
 
@@ -23,6 +24,12 @@ def __init__(self, grid: GridObject):
         ----------
         grid : GridObject
             The GridObject that will be the basis of the computation.
+        bc : ndarray or GridObject, optional
+            Boundary conditions for sink filling. `bc` should be an array
+            of np.uint8 that matches the shape of the DEM. Values of 1
+            indicate pixels that should be fixed to their values in the
+            original DEM and values of 0 indicate pixels that should be
+            filled.
 
         Notes
         -----
@@ -33,7 +40,28 @@ def __init__(self, grid: GridObject):
         dem = grid.z
 
         filled_dem = np.zeros_like(dem, dtype=np.float32, order='F')
-        _grid.fillsinks(filled_dem, dem, dims)
+        restore_nans = False
+        if bc is None:
+            bc = np.ones_like(dem, dtype=np.uint8)
+            bc[1:-1, 1:-1] = 0  # Set interior pixels to 0
+
+            nans = np.isnan(dem)
+            dem[nans] = -np.inf
+            bc[nans] = 1
+            restore_nans = True
+
+        if bc.shape != dims:
+            err = ("The shape of the provided boundary conditions does not "
+                   f"match the shape of the DEM. {dims}")
+            raise ValueError(err)from None
+
+        if isinstance(bc, GridObject):
+            bc = bc.z
+
+        _grid.fillsinks(filled_dem, dem, bc, dims)
+
+        if restore_nans:
+            filled_dem[nans] = np.nan
 
         flats = np.zeros_like(dem, dtype=np.int32, order='F')
         _grid.identifyflats(flats, filled_dem, dims)
diff --git a/src/topotoolbox/grid_object.py b/src/topotoolbox/grid_object.py
index 82bdf62..6e249c7 100755
--- a/src/topotoolbox/grid_object.py
+++ b/src/topotoolbox/grid_object.py
@@ -38,19 +38,52 @@ def __init__(self) -> None:
         self.transform = None
         self.crs = None
 
-    def fillsinks(self) -> 'GridObject':
+    def fillsinks(self,
+                  bc: 'np.ndarray | GridObject | None' = None) -> 'GridObject':
         """Fill sinks in the digital elevation model (DEM).
 
+        Parameters
+        ----------
+        bc : ndarray or GridObject, optional
+            Boundary conditions for sink filling. `bc` should be an array
+            of np.uint8 that matches the shape of the DEM. Values of 1
+            indicate pixels that should be fixed to their values in the
+            original DEM and values of 0 indicate pixels that should be
+            filled.
+
         Returns
         -------
         GridObject
             The filled DEM.
+
         """
 
         dem = self.z.astype(np.float32, order='F')
         output = np.zeros_like(dem)
 
-        _grid.fillsinks(output, dem, self.shape)
+        restore_nans = False
+
+        if bc is None:
+            bc = np.ones_like(dem, dtype=np.uint8)
+            bc[1:-1, 1:-1] = 0  # Set interior pixels to 0
+
+            nans = np.isnan(dem)
+            dem[nans] = -np.inf
+            bc[nans] = 1  # Set NaNs to 1
+            restore_nans = True
+
+        if bc.shape != self.shape:
+            err = ("The shape of the provided boundary conditions does not "
+                   f"match the shape of the DEM. {self.shape}")
+            raise ValueError(err)from None
+
+        if isinstance(bc, GridObject):
+            bc = bc.z
+
+        _grid.fillsinks(output, dem, bc, self.shape)
+
+        if restore_nans:
+            output[nans] = np.nan
 
         result = copy.copy(self)
         result.z = output