What follows is a brief summary of the key points of Mr. Schwartz’s presentation at the Annual Meeting Thursday evening. I will try to get some picture or sketches to illustrate my notes, but I want to provide something for those who were not able to join us.
A river system with stable dynamics will have four characteristics:
A good riparian corridor – banks covered with trees and bushes.
Lateral stability – no bare banks, no tendency to erode the banks.
Sediment transport competence – the sufficient volume and velocity of water to move sediment and deposit sediment throughout the stream system, and sufficient internal structure to store it.
Flood plain access – this allows for return to substantially the same channel after high water.
These four characteristics interact with and reinforce each other in a positive way when all are good, resulting in a stream system that is healthy for people and their structures as well as healthy for trout habitat and other wildlife. The stream flow interacts with the structure of the stream bed through alterations of fast water and slow water: fast water always picks up sediment, and slow water drops it. After a big flood the river returns to essentially the same channel. In steep streams the internal structure of large stones creates alterations of slow and fast water to dissipate energy and store sediment as waterfalls and pools form over the rocks. In streams with shallow slopes side to side curves or meanders allow the deposition of sediment in bars, with deep curving currents around them.
But when one or more of these components is absent, they can create the vicious cycle of erosion, excess sedimentation, and channel cutting that we have seen so often. The dynamics and hydrology of the stream continue to try to create the vertical and horizontal structures, the sediment deposits, and the flood plains that is required for stability, but the system is out of balance.
Mr. Schwartz said that the common past practice of channelization disrupts all four of these stabilizing factors, and he highlighted the problems with sediment transport in particular, as this is not always well understood.
Channelization is when a more or less rectangular channel is excavated for a stream that is intended to be big enough to contain flood flows, so that the water does not come up into what would otherwise be the flood plain. These channels are often also relatively straight, to keep a stream away from roads, or farm fields, or houses. Landowners generally like channelization, because they think that they are keeping the floodwaters off of their land, and just moving the water downstream. But channelization only works when it doesn’t rain. When it really rains, channelized streams mean that some landowners will essentially send some of their land downstream through erosion. So the upstream landowers will lose some of their land, and the downstream landowners will get sediment deposits that cause problems for them.
If the channelized stream is steep, in high water a kind of firehouse effect is created that focuses tremendous volume and velocity of water on the banks in places, often resulting in bank failure and erosion, with resulting excess sedimentation downstream. Even without massive bank failure, the force of the water will scour sediment from the banks and the bottom, sending it downstream. The dynamics of the channelized steep stream keep trying to create some kind of flood plain shelf through the bank failures and debris blockages, but it just keeps getting deeper and deeper.
If the channelized stream is shallow in slope, in ordinary flows the current will be too wide and shallow to move the gravel or cobble sediment. Depositions of sediment will tend to form islands in the center of the channel, with the deepest and most intense flows on either side. Therefore in high water the erosive force of the current will be targeted towards either or both banks, resulting in erosion and bank failure despite the size of the channel. This will then increase the deposits of sediment downstream, with more erosion and channel cutting as a result. A flat stream does not what to go straight, and it will keep trying to make curves through the interactions of water and the sediment carried.
There can be particularly severe problems during flooding where a steep channelized stream turns into a flat channelized stream as it comes out of the hills into the valley. Or at the confluence of two channelized streams of any kind. There are likely to be massive deposits of sediment as the flood waters meet, slow down, and drop their sediment. There is likely to be cutting of new channels due to sediment and debris blockages. When such a confluence is in a town or village with roads and bridges surrounding it, tremendous damage can be done.
The answer is to try to move towards the four conditions for stable river dynamics listed above. Wherever possible we need to provide flood plain access rather than trying to keep streams within a channel. Wherever possible we need to give the streams some room to make curves and move around within their corridor. And we need to plant the banks with trees and bushes wherever possible. This could result in a stream system that can deposit sediment, form bars, form curves, form pools, and then move them and recreate them elsewhere without causing damage. By giving the streams more room to work out the balance between the water and the sediment, we may achieve a more stable dynamics that will be good for fish and good for people.
But to get this we need to give the rivers more room where we can. That is the message of Irene. All of us throughout the watershed have to take responsibility for this.