Preheat oven to 350 degrees F. Grease two 9x5-inch loaf pans.
Make the Cinnamon Crumb Topping: In a medium bowl, whisk together the flour, sugars, cinnamon and salt. Add the butter and mix with a rubber spatula until the mixture comes together into large, coarse crumbs. Cover and refrigerate until ready to use.
Make the Cinnamon-Maple Walnuts: In a medium bowl, stir together the walnuts, maple syrup, brown sugar and cinnamon until the walnuts are evenly coated. Set aside until ready to use.
In a large bowl, whisk together the sugar, oil, pumpkin, water, eggs and vanilla extract. Add both flours, baking soda, baking powder, salt and nutmeg, and use a rubber spatula to gently stir until completely combined.
Pour one-quarter of the batter into each of the prepared loaf pans. Divide the cinnamon-maple walnuts between each of the loaf pans, sprinkling them into an even layer on top of the batter. Divide the remaining batter between the loaf pans, covering the walnut filling. Sprinkle the crumb topping evenly over the top of each loaf, pressing lightly so the crumbs adhere to the batter.
Bake for 60 to 80 minutes, or until a cake tester or toothpick inserted in the center comes out clean. Remove from the oven and cool in the pans for 10 minutes, then turn out to a wire rack to cool completely. Once the loaves are completely cool, wrap tightly in plastic wrap, and store overnight (at room temperature) before serving. Leftovers can be stored at room temperature, wrapped tightly in plastic wrap, for up to 4 days. This bread can also be frozen for up to 2 months.