Landslide dams (dammed lakes) posses distinct geomorphologic and environmental effects, which can be grouped to short-term and long-term on time-scale, and mainly embody on hydrologic process, fluvial morphology, environment and ecology, and landscape. During the initial stage of landslide dams, the aquatic ecology and landscape often undergo some kinds of degradation since the pre-existing habitats were violently disturbed or even destroyed. Burst of landslide dams intensively change the hydrological process and fluvial morphology within a short time, and have strong negative impacts, even devastation, on downstream aquatic habitats, ecology and landscape. The landslide dams (dammed lakes) may be preserved for long term, and they will develop into knickpoints by long-term action of the flow, and act as a primary control of river bed incision, thus, reducing the potential of new landslide. Landslide dams (dammed lakes) which stably maintain in long-term scale greatly affect the fluvial morphology. Large knickpoints can totally change the fluvial processes and river morphology. Uplift of the Qinghai-Tibetan Plateau has caused extensive channel bed incision along almost all rivers. For many rivers (such as the upstream Yangtze) the incision has been partly controlled by knickpoints. Upstream reaches of a knickpoint have a new and unchanging base level. If hundreds of landslide dams occurred simultaneously on a reach of a mountain river, the potential energy of bank failure and the slope erosion would be greatly reduced and sediment yield from the watershed might be reduced significantly. The long-term maintained dammed lakes can greatly enhance stream habitats, aquatic ecology, surrounding landscape, and sometimes cause a change in local micro-climate. Many dammed lakes have become favorable touring destination, and have potentiality for hydropower generation. Landslide dams (dammed lakes) are knickpoints formed by natural feedback to continually incision of stream bed and instability of bank slope, and they can increase flow resistance and control streambed incision, and is a promotional factor to river stability and health.