Dam Sediments: Over, Under or Through?

Two thirds of freshwater flowing to the ocean is obstructed by more than 45,000 large dams worldwide. There are roughly 2.5 million small dams ( <35 feet in height) in the United States alone. Hence most riverine systems into which stormwater and sediments flow have been fundamentally altered. For example, river basin hydrology and fluvial-geomorphology have been radically changed throughout mid Atlantic watersheds in the U.S. as a result of 18th century mill dams. As a result, the reference conditions used to develop the metrics for watershed health and serve as models for water rehabilitation/restoration are often based on current altered and often severely degraded watershed conditions.

In this session we will explore the primary structure and functions of rivers including water storage and conveyance and sediment transport. We will discuss different types of dams, the benefits of dams, where they are built, and then focus in on the lasting impact of dams have on sediment dynamics, channel form, hydrology and water quality, instream biological communities, and riparian vegetation. The concept of “hungry water” in sediment-starved rivers will be discussed. In particular, the impacts of dams on the transport of suspended load and bedload from headwaters to river mouth will be elucidated as will methods to mimic or restore sediment dynamics found in less-altered natural systems. Lastly, we will explore some of the drivers of dam removal in the U.S. and removal methods and watershed recovery.

This session is a must for engineers, resource managers, stormwater specialists, and developers who work in watersheds with dams and reservoirs who want to better understand the impacts and management options.