Final answer:
The sediment dam formed by the eruption of Mount St. Helens altered the landscape by blocking streams and rivers, resulting in the formation of new lakes.
Step-by-step explanation:
The Mount St. Helens Sediment Dam was not specifically constructed. The sediment dam in the question refers to the natural feature formed by the volcanic eruption and subsequent landslide of Mount St. Helens in 1980.
During the eruption, the north flank of Mount St. Helens collapsed, resulting in a massive landslide that wiped out all vegetation on the northern side of the mountain. This landslide, along with the volcanic ash and other materials, created a large amount of sediment that blocked streams and rivers, forming what is known as a sediment dam.
The sediment dam altered the landscape and caused water to back up, resulting in the formation of new lakes, such as Spirit Lake. These lakes continue to exist today and are the result of the sediment dam formed by the eruption of Mount St. Helens.