Sand dams are small, reinforced barriers constructed across seasonal and ephemeral streams which trap water in sediments deposited. For these reservoirs to provide sustainable and dependable water supplies or valuable sand for other purposes, they should primarily fill with coarse sand rather than fine sediments. Excessive accumulation of fine sediments in sand dam reservoirs limits recharge and recoverable water. We describe a novel approach to preventing the accumulation of fine sediments in sand dam reservoirs by geomorphic management of reservoir sedimentation. We propose building sand dams with outlets at the foot of the dam to selectively trap coarse sediments (> 0.125 mm; Rouse number = 2.5) across a range of flows and sediment transport rates. An optimal outlet had an “Eiffel Tower” shape which maintains the desired Rouse number, ensuring finer particles will pass out of the reservoir remaining suspended, while coarser particles settle. HEC-RAS simulations confirm that these designs promote uniform coarse sediment deposition within the reservoir and perform effectively, with minimal deviation from the target Rouse number, with a mean squared error (MSE) of less than 1%. Alternative rectangular and circular base cutouts which can be readily made by embedding culvert pipe also performed well across a wide range of flows. These shapes are simpler to construct and maintain greater structural integrity than the more complex Eiffel Tower (ET) shape. While the ET shape gave the most consistent performance in our tests, simpler designs may offer a better balance between performance, ease of construction, and strength