The presence of woody debris, such as trees and shrubs, in and adjacent to rivers and streams has important effects on the riparian system of stream channels. Large concentrations of woody debris can cause jams which can lead to devastating flooding and damage to infrastructures in and along the channel. Woody debris jams can also have important effects on channel morphology, including shoreline erosion, scour and deposition of sediment, lateral migration of channels, and shifting of flow in branching channels. In addition to affecting the hydraulics and morphology of stream channels, the presence of woody debris can greatly influence aquatic ecology by affecting fish habitat and nutrient fluxes. Although the influence of wood debris on channel morphology and aquatic ecology has been well recognized, the understanding on the mechanics of woody debris jams is very limited. In this paper, the results of a laboratory investigation flume study on physical properties of woody jams and the effects of jams on channel hydraulics using poplar dowels will be summarized. The theory on woody debris jam as an accumulation of floating logs based on the flume experiments is developed. A numerical model for woody debris jams will be presented. The modeled jam thickness profiles and effects of jam on flow conditions, including the backwater produced by the jam compared well with the flume data.