River Response to Disturbances: The behavior of a river is largely determined by four controlling variables:
The volume (discharge) of water typically flowing through the channel = Q
The amount (load) of sediment entering the river = L
The average size (diameter) of that sediment (small-fine vs large-course) = D
The slope of the river = S
A change in any of these controlling factors can cause river aggradation or degradation. Aggradation is the buildup of alluvium (river transported sediment) which causes the river channel to rise. Degradation is the lowering of the channel caused by river erosion. Aggradation can damage nearby structures by burying them. Degradation can damage nearby structures by undermining foundations.
Lane's balance (illustrated below) shows how changes in discharge (Q), sediment load (L), sediment diameter (D), and channel slope (S) can interact to cause either river aggradation or degradation. For example, if the river sediments became finer (i.e. smaller D) than the tray on the left side of the balance would shift to the right causing the balance to tip to the right. This would cause the needle on the balance to move into the degradation side, meaning that degradation would likely result if all of the other variables remained unchanged. Another example would be if there was a decrease in discharge (i.e. lower Q). Less water in the bucket on the right side of the balance would cause the balance to tip to the left and the needle would move into the aggredation side, so aggredation would be the result if the other variables remained unchanged.
Using Lane's balance as a guide, indicate what the river response would be for the scenarios listed below. Possible answers are degradation, aggradation, or negating effects. Negating effects means that the disturbance may cause both aggradation and degradation so the effects may partially or totally cancel each other out, so it is uncertain what the response may be.
A landslide delivered a large amount of coarse sediment into the river
A water diversion project started to remove water from a river channel for irrigation
Volcanic or tectonic uplift caused a river channel to become steeper
Climate change in a region caused greater rainfall resulting in higher steam discharges
A land stabilization project has reduced the amount of sediment entering a river channel
A construction project has steepened the slope of a river and decreased the amount of sediment entering the channel
The building of a dam has decreased the amount of sediment and decreased the amount of water flowing into the channel downstream
The clearing of a forest has increased both the amount of water and sediment entering a river channel
A river is diverted into a new channel that is less steep and has a greater supply of coarse sediment
A river is diverted into a new channel that is steeper and has a greater supply of coarse sediment
Longer Answer Questions
Climate Change and U.S. Coastal Areas: At this website (https://www3.epa.gov/climatechange/impacts/coasts.html) you will find a write-up on the observed and anticipated effects of climate change on coastal zones prepared by the US EPA (Environmental Protection Agency). Read this article and answer the questions below.
What are the major impacts of climate change on coastal areas discussed in this reading?
For the impact of sea level rise, why is it helpful to distinguish between "local" or "relative" sea level change and "global" sea level change?
What would your response be to rising sea levels if you owned a beach house that was being threatened? Briefly explain.