Abstract: Apparatus for monitoring the amount of sediment carried into and deposited within an underground stormwater-receiving system comprises a high surface area float that moves up and down within a chamber of the system. A rod extends upwardly from the float and is slidably contained within a tube that runs from the top of the chamber and to the soil surface. The top of the rod is visible to a worker on the surface. When storm water drains away, the float comes to rest on such sediment as has accumulated in the bottom of the chamber. Thus, the amount of sediment is determinable according to the changed rest-position of the rod.