As fine sediments are transported down the river bottom they may come to rest and are often packed in the spaces between the larger rocks. These larger rocks are then said to be "embedded" in the finer sediments. Embeddedness is measured as the percentage to which a rock on the river bottom is buried, or embedded, in the finer materials. For example, a rock which is 90% embedded is almost completely covered by the embedding material.
From their experience with the trout eggs, student know that this embedding material may mean that the trout will not be able to lay their eggs. According to fisheries biologists trout spawn in river bottoms where the velocity of the water is fast enough to ensure adequate aeration of the bottom gravels. But if the bottom rocks are more than 30% embedded they are poor habitat for nesting trout and values of 50% or more indicate that the fish may not lay eggs at all. With this much embedding material present the eggs will be smothered by the sediment.
High levels of embeddedness also affect the stream macroinvertebrates. While some species can cope with this type of pollution, it seems that the species richness declines when the embeddedness is high. These organisms form an important source of food for the young trout and their disappearance indicates that the stream is a poor habitat for trout.
The field study Embeddedness provides a worksheet that shows students how to calculate the average embeddedness of a stream bottom.