Unconsolidated Alluvium Figure 2. Unconsolidated alluvium in a quarry pit near Eagle, Idaho. In Idaho, these sediments are frequently found in the river and stream valleys between mountains. Unconsolidated alluvium aquifers occur commonly in northern, central and southern Idaho. However, it is not unusual to find small unconsolidated alluvium aquifers in many places throughout the state.
Alluvial aquifers are the result of erosion.
Rocks are typically eroded from the mountains and deposited by streams and rivers in lowlands such as plains and valleys. The sediments are unconsolidated which means they have not been compacted and cemented into rocks. Alluvial sediments can be only a few feet thick or hundreds of feet in thickness. The sediments may include boulders, cobbles, gravel, sand, silt, and clay. And often, these sediment are aquifers because they contain ground water that can be withdrawn and used.