diff --git a/scripts/train.py b/scripts/train.py
index fcbe9d0..99a53ef 100644
--- a/scripts/train.py
+++ b/scripts/train.py
@@ -8,7 +8,6 @@
 import torch
 from hydra.core.hydra_config import HydraConfig
 from omegaconf import DictConfig
-from torch.nn.functional import mse_loss
 from torch.utils.data import DataLoader, RandomSampler
 
 from ddr import ddr_functions, dmc, kan, streamflow
@@ -92,7 +91,7 @@ def train(cfg: Config, flow: streamflow, routing_model: dmc, nn: kan) -> None:
 
                 filtered_predictions = daily_runoff[~np_nan_mask]
 
-                loss = mse_loss(
+                loss = torch.nn.functional.l1_loss(
                     input=filtered_predictions.transpose(0, 1)[cfg.experiment.warmup :].unsqueeze(2),
                     target=filtered_observations.transpose(0, 1)[cfg.experiment.warmup :].unsqueeze(2),
                 )
@@ -120,8 +119,7 @@ def train(cfg: Config, flow: streamflow, routing_model: dmc, nn: kan) -> None:
                 param_map = {
                     "n": routing_model.n,
                     "q_spatial": routing_model.q_spatial,
-                    "top_width": routing_model.top_width,
-                    "side_slope": routing_model.side_slope,
+                    "p_spatial": routing_model.p_spatial,
                 }
                 for param_name in cfg.kan.learnable_parameters:
                     param_tensor = param_map.get(param_name)
@@ -155,6 +153,13 @@ def train(cfg: Config, flow: streamflow, routing_model: dmc, nn: kan) -> None:
                     saved_model_path=cfg.params.save_path / "saved_models",
                 )
 
+                # Free autograd graph and routing engine tensors to prevent OOM
+                del dmc_output, daily_runoff, streamflow_predictions, loss
+                del filtered_predictions, filtered_observations
+                routing_model.routing_engine.q_prime = None
+                routing_model.routing_engine._discharge_t = None
+                torch.cuda.empty_cache()
+
 
 @hydra.main(
     version_base="1.3",
diff --git a/src/ddr/routing/mmc.py b/src/ddr/routing/mmc.py
index 7802839..b51fd66 100644
--- a/src/ddr/routing/mmc.py
+++ b/src/ddr/routing/mmc.py
@@ -71,36 +71,68 @@ def _log_base_q(x: torch.Tensor, q: float) -> torch.Tensor:
     return torch.log(x) / torch.log(torch.tensor(q, dtype=x.dtype))
 
 
+def _apply_data_override(derived: torch.Tensor, data: torch.Tensor | None) -> torch.Tensor:
+    """Override derived values with observed data where available.
+
+    Three cases:
+    1. data is None or empty -> return derived (MERIT without observed geometry)
+    2. data has no NaN -> return data (Lynker full coverage)
+    3. data has NaN -> blend: data where valid, derived where NaN (partial coverage)
+
+    Parameters
+    ----------
+    derived : torch.Tensor
+        Values derived from the power law
+    data : torch.Tensor or None
+        Observed data, possibly containing NaN
+
+    Returns
+    -------
+    torch.Tensor
+        Blended result
+    """
+    if data is None or data.numel() == 0:
+        return derived
+    nan_mask = torch.isnan(data)
+    if not nan_mask.any():
+        return data
+    return torch.where(~nan_mask, data, derived)
+
+
 def _get_trapezoid_velocity(
     q_t: torch.Tensor,
     _n: torch.Tensor,
-    top_width: torch.Tensor,
-    side_slope: torch.Tensor,
     _s0: torch.Tensor,
     p_spatial: torch.Tensor,
     _q_spatial: torch.Tensor,
+    data_top_width: torch.Tensor | None,
+    data_side_slope: torch.Tensor | None,
     velocity_lb: torch.Tensor,
     depth_lb: torch.Tensor,
     _btm_width_lb: torch.Tensor,
-) -> torch.Tensor:
+) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
     """Calculate flow velocity using Manning's equation for trapezoidal channels.
 
+    Derives top_width and side_slope per-timestep from the Leopold & Maddock
+    power law (top_width = p * depth^q), then overrides with observed data
+    where available.
+
     Parameters
     ----------
     q_t : torch.Tensor
         Discharge at time t
     _n : torch.Tensor
         Manning's roughness coefficient
-    top_width : torch.Tensor
-        Top width of channel
-    side_slope : torch.Tensor
-        Side slope of channel (z:1, z horizontal : 1 vertical)
     _s0 : torch.Tensor
         Channel slope
     p_spatial : torch.Tensor
-        Spatial parameter p
+        Leopold & Maddock width coefficient
     _q_spatial : torch.Tensor
-        Spatial parameter q
+        Leopold & Maddock width-depth exponent
+    data_top_width : torch.Tensor or None
+        Observed top width data for override (Lynker/SWOT), or None/empty
+    data_side_slope : torch.Tensor or None
+        Observed side slope data for override (Lynker/SWOT), or None/empty
     velocity_lb : torch.Tensor
         Lower bound for velocity
     depth_lb : torch.Tensor
@@ -110,19 +142,27 @@ def _get_trapezoid_velocity(
 
     Returns
     -------
-    torch.Tensor
-        Flow velocity
+    tuple[torch.Tensor, torch.Tensor, torch.Tensor]
+        (celerity, top_width, side_slope)
     """
-    numerator = q_t * _n * (_q_spatial + 1)
+    q_eps = _q_spatial + 1e-6
+    numerator = q_t * _n * (q_eps + 1)
     denominator = p_spatial * torch.pow(_s0, 0.5)
     depth = torch.clamp(
         torch.pow(
             torch.div(numerator, denominator + 1e-8),
-            torch.div(3.0, 5.0 + 3.0 * _q_spatial),
+            torch.div(3.0, 5.0 + 3.0 * q_eps),
         ),
         min=depth_lb,
     )
 
+    # Derive top_width and side_slope from Leopold & Maddock power law
+    top_width = p_spatial * torch.pow(depth, q_eps)
+    top_width = _apply_data_override(top_width, data_top_width)
+
+    side_slope = torch.clamp(top_width * q_eps / (2 * depth), min=0.5, max=50.0)
+    side_slope = _apply_data_override(side_slope, data_side_slope)
+
     # For z:1 side slopes (z horizontal : 1 vertical)
     _bottom_width = top_width - (2 * side_slope * depth)
     bottom_width = torch.clamp(_bottom_width, min=_btm_width_lb)
@@ -140,7 +180,7 @@ def _get_trapezoid_velocity(
     v = torch.div(1, _n) * torch.pow(R, (2 / 3)) * torch.pow(_s0, (1 / 2))
     c_ = torch.clamp(v, min=velocity_lb, max=torch.tensor(15.0, device=v.device))
     c = c_ * 5 / 3
-    return c
+    return c, top_width, side_slope
 
 
 class MuskingumCunge:
@@ -186,8 +226,10 @@ def __init__(self, cfg: Config, device: str | torch.device = "cpu") -> None:
         self.routing_dataclass: Any = None
         self.length: torch.Tensor | None = None
         self.slope: torch.Tensor | None = None
-        self.top_width: torch.Tensor | None = None
-        self.side_slope: torch.Tensor | None = None
+        self.top_width: torch.Tensor | None = None  # Derived per-timestep in route_timestep()
+        self.side_slope: torch.Tensor | None = None  # Derived per-timestep in route_timestep()
+        self._data_top_width: torch.Tensor | None = None  # Observed data for override
+        self._data_side_slope: torch.Tensor | None = None  # Observed data for override
         self.x_storage: torch.Tensor | None = None
         self.observations: Any = None
         self.output_indices: list[Any] | None = None
@@ -261,6 +303,16 @@ def _set_network_context(self, routing_dataclass: Any, streamflow: torch.Tensor)
         )
         self.x_storage = routing_dataclass.x.to(self.device).to(torch.float32)
 
+        # Store observed geometry for data override in _get_trapezoid_velocity
+        if routing_dataclass.top_width is not None and routing_dataclass.top_width.numel() > 0:
+            self._data_top_width = routing_dataclass.top_width.to(self.device).to(torch.float32)
+        else:
+            self._data_top_width = None
+        if routing_dataclass.side_slope is not None and routing_dataclass.side_slope.numel() > 0:
+            self._data_side_slope = routing_dataclass.side_slope.to(self.device).to(torch.float32)
+        else:
+            self._data_side_slope = None
+
         self.q_prime = streamflow.to(self.device)
 
         if routing_dataclass.flow_scale is not None:
@@ -282,23 +334,13 @@ def _denormalize_spatial_parameters(self, spatial_parameters: dict[str, torch.Te
             log_space="q_spatial" in log_space_params,
         )
 
-        routing_dataclass = self.routing_dataclass
-        if routing_dataclass.top_width.numel() == 0:
-            self.top_width = denormalize(
-                value=spatial_parameters["top_width"],
-                bounds=self.parameter_bounds["top_width"],
-                log_space="top_width" in log_space_params,
+        # p_spatial: use learned value if in spatial_parameters, otherwise use default
+        if "p_spatial" in spatial_parameters and "p_spatial" in self.parameter_bounds:
+            self.p_spatial = denormalize(
+                value=spatial_parameters["p_spatial"],
+                bounds=self.parameter_bounds["p_spatial"],
+                log_space="p_spatial" in log_space_params,
             )
-        else:
-            self.top_width = routing_dataclass.top_width.to(self.device).to(torch.float32)
-        if routing_dataclass.side_slope.numel() == 0:
-            self.side_slope = denormalize(
-                value=spatial_parameters["side_slope"],
-                bounds=self.parameter_bounds["side_slope"],
-                log_space="side_slope" in log_space_params,
-            )
-        else:
-            self.side_slope = routing_dataclass.side_slope.to(self.device).to(torch.float32)
 
     def _init_discharge_state(self, carry_state: bool) -> None:
         """Cold-start via topological accumulation, or carry from previous batch."""
@@ -373,15 +415,15 @@ def forward(self) -> torch.Tensor:
                 dtype=torch.float32,
             )
 
-            # Vectorized initial values
+            # Vectorized initial values (avoid double in-place write for autograd safety)
             gathered = self._discharge_t[self._flat_indices]
-            output[:, 0] = torch.scatter_add(
+            initial = torch.scatter_add(
                 input=self._scatter_input,
                 dim=0,
                 index=self._group_ids,
                 src=gathered,
             )
-            output[:, 0] = torch.clamp(output[:, 0], min=self.discharge_lb)
+            output[:, 0] = torch.clamp(initial, min=self.discharge_lb)
 
         # Route through time series
         for timestep in tqdm(
@@ -478,8 +520,6 @@ def route_timestep(
         if (
             self._discharge_t is None
             or self.n is None
-            or self.top_width is None
-            or self.side_slope is None
             or self.slope is None
             or self.q_spatial is None
             or self.length is None
@@ -488,15 +528,15 @@ def route_timestep(
         ):
             raise ValueError("Required attributes not set. Call setup_inputs() first.")
 
-        # Calculate velocity using internal routing_dataclass data
-        velocity = _get_trapezoid_velocity(
+        # Calculate velocity and derive top_width/side_slope from Leopold & Maddock power law
+        velocity, self.top_width, self.side_slope = _get_trapezoid_velocity(
             q_t=self._discharge_t,
             _n=self.n,
-            top_width=self.top_width,
-            side_slope=self.side_slope,
             _s0=self.slope,
             p_spatial=self.p_spatial,
             _q_spatial=self.q_spatial,
+            data_top_width=self._data_top_width,
+            data_side_slope=self._data_side_slope,
             velocity_lb=self.velocity_lb,
             depth_lb=self.depth_lb,
             _btm_width_lb=self.bottom_width_lb,
diff --git a/src/ddr/routing/torch_mc.py b/src/ddr/routing/torch_mc.py
index ac89068..b6034fd 100644
--- a/src/ddr/routing/torch_mc.py
+++ b/src/ddr/routing/torch_mc.py
@@ -180,13 +180,16 @@ def forward(self, **kwargs: Any) -> dict[str, torch.Tensor]:
         self.network = self.routing_engine.network
         self.n = self.routing_engine.n
         self.q_spatial = self.routing_engine.q_spatial
-        self.top_width = self.routing_engine.top_width
-        self.side_slope = self.routing_engine.side_slope
+        self.p_spatial = self.routing_engine.p_spatial
         self._discharge_t = self.routing_engine._discharge_t
 
         # Perform routing
         output = self.routing_engine.forward()
 
+        # Read back per-timestep derived geometry from the engine
+        self.top_width = self.routing_engine.top_width
+        self.side_slope = self.routing_engine.side_slope
+
         # Update discharge state for compatibility
         self._discharge_t = self.routing_engine._discharge_t
 
@@ -195,6 +198,8 @@ def forward(self, **kwargs: Any) -> dict[str, torch.Tensor]:
                 self.n.retain_grad()
             if self.q_spatial is not None:
                 self.q_spatial.retain_grad()
+            if self.p_spatial is not None and self.p_spatial.requires_grad:
+                self.p_spatial.retain_grad()
             if self._discharge_t is not None:
                 self._discharge_t.retain_grad()
 
@@ -205,10 +210,6 @@ def forward(self, **kwargs: Any) -> dict[str, torch.Tensor]:
                     spatial_params["n"].retain_grad()
                 if "q_spatial" in spatial_params:
                     spatial_params["q_spatial"].retain_grad()
-                if "top_width" in spatial_params:
-                    spatial_params["top_width"].retain_grad()
-                if "side_slope" in spatial_params:
-                    spatial_params["side_slope"].retain_grad()
                 if "p_spatial" in spatial_params:
                     spatial_params["p_spatial"].retain_grad()
 
diff --git a/src/ddr/validation/configs.py b/src/ddr/validation/configs.py
index 61c773c..301ad12 100644
--- a/src/ddr/validation/configs.py
+++ b/src/ddr/validation/configs.py
@@ -92,15 +92,13 @@ class Params(BaseModel):
         default_factory=lambda: {
             "n": [0.015, 0.25],  # (m⁻¹/³s)
             "q_spatial": [0.0, 1.0],  # 0 = rectangular, 1 = triangular
-            "top_width": [1.0, 5000.0],  # Log-space (m)
-            "side_slope": [0.5, 50.0],  # H:V ratio Log-space (-)
+            "p_spatial": [1.0, 200.0],  # Leopold & Maddock width coefficient, Log-space (m)
         },
         description="The parameter space bounds [min, max] to project learned physical values to",
     )
     log_space_parameters: list[str] = Field(
         default_factory=lambda: [
-            "top_width",
-            "side_slope",
+            "p_spatial",
         ],
         description="Parameters to denormalize in log-space for right-skewed distributions",
     )
diff --git a/tests/routing/test_mmc.py b/tests/routing/test_mmc.py
index 2dd8e5d..4bb1d3c 100644
--- a/tests/routing/test_mmc.py
+++ b/tests/routing/test_mmc.py
@@ -60,14 +60,11 @@ def test_setup_inputs_basic(self) -> None:
         # Check spatial attributes
         assert mc.length is not None
         assert mc.slope is not None
-        assert mc.top_width is not None
-        assert mc.side_slope is not None
         assert mc.x_storage is not None
         assert_tensor_properties(mc.length, (10,))
         assert_tensor_properties(mc.slope, (10,))
-        assert_tensor_properties(mc.top_width, (10,))
-        assert_tensor_properties(mc.side_slope, (10,))
         assert_tensor_properties(mc.x_storage, (10,))
+        # top_width and side_slope are now derived per-timestep in route_timestep()
 
         # Check parameter denormalization
         assert mc.n is not None
@@ -115,16 +112,13 @@ def test_setup_inputs_device_conversion(self) -> None:
         # Check that all tensors are on correct device
         assert mc.length is not None
         assert mc.slope is not None
-        assert mc.top_width is not None
-        assert mc.side_slope is not None
         assert mc.x_storage is not None
         assert mc.q_prime is not None
         assert mc.length.device.type == "cpu"
         assert mc.slope.device.type == "cpu"
-        assert mc.top_width.device.type == "cpu"
-        assert mc.side_slope.device.type == "cpu"
         assert mc.x_storage.device.type == "cpu"
         assert mc.q_prime.device.type == "cpu"
+        # top_width and side_slope are derived per-timestep in route_timestep()
 
 
 class TestMuskingumCungeSparseOperations:
diff --git a/tests/routing/test_utils.py b/tests/routing/test_utils.py
index 9c4ba17..53a0489 100644
--- a/tests/routing/test_utils.py
+++ b/tests/routing/test_utils.py
@@ -42,7 +42,7 @@ def create_mock_config() -> Config:
             "gages": "mock.csv",
         },
         "params": {
-            "parameter_ranges": {"n": [0.01, 0.1], "q_spatial": [0.1, 0.9]},
+            "parameter_ranges": {"n": [0.01, 0.1], "q_spatial": [0.1, 0.9], "p_spatial": [1.0, 200.0]},
             "defaults": {"p_spatial": 1.0},
             "attribute_minimums": {
                 "velocity": 0.1,
diff --git a/tests/validation/test_configs.py b/tests/validation/test_configs.py
index 9610452..2b235e3 100644
--- a/tests/validation/test_configs.py
+++ b/tests/validation/test_configs.py
@@ -136,12 +136,11 @@ def test_parameter_ranges_defaults(self) -> None:
         p = Params()
         assert "n" in p.parameter_ranges
         assert "q_spatial" in p.parameter_ranges
-        assert "top_width" in p.parameter_ranges
-        assert "side_slope" in p.parameter_ranges
+        assert "p_spatial" in p.parameter_ranges
 
     def test_log_space_parameters_default(self) -> None:
         p = Params()
-        assert p.log_space_parameters == ["top_width", "side_slope"]
+        assert p.log_space_parameters == ["p_spatial"]
 
 
 class TestSetSeed: