Key River Science Insights
These insights are part of our report Supporting New England Communities to Become River-Smart: Policies and programs that can help New England towns thrive despite river floods
- Rivers carry more than water, and rivers in flood are not simply a rise in water elevation. Rivers in flood carry and move large volumes of sediment and debris, and travel with tremendous velocity and force.
- Small streams can swell enormously during floods, rising much higher than their banks.
- High, fast, powerful flood flows can rapidly erode, undercut, and carry away parts of their landscapes.
- The material eroded by fast-moving floodwaters is deposited somewhere else wherever the river slows down and spreads out -in floodplains, in the inside of river bends, in flatter more open valleys far downstream, or even in people's homes.
- Human land-based activities often accelerate the movement or sediments, soils, debris, and even parts of the landscape from hill slope to valley by making material easier to mobilize. This effect can last decades or centuries.
- If rivers are allowed to flood, and spread out to their floodplains when they flood, they contribute important nutrients and ecological connectivity. Spreading waters into the floodplain also lessens the force and damage of the river flood for those downstream.
- It is in the nature of rivers to move their channels and change their landscapes. This is a constant but highly variable process with some predictable patterns.
- Floodplains are formed by rivers. If a road or structure is on a floodplain then it resides in a place where the river has run or flooded in the past, and is likely to do so again.
- When people put obstacles in the way of rivers so they cannot access their floodplains, the force of a river flood may break through the obstacles. Alternatively, if the raging river cannot break through, its full force will be retained as it rushes downstream. Either way, the result is often disastrous to human-built structures.
- We cannot know exactly where rivers will move, erode, or deposit sediment or debris, but with an understanding and assessment of specific river processes, patterns and features, we can identify places of high risk.
- Straightened, confined rivers are faster and more powerful.
- Straightened, confined rivers especially when they have been excavated, tend to down-cut their beds. A river that has down-cut often then re-widens at a lower level. This is likely to be destructive of levees, dikes, berms and other protective structures, as well as the investments they were built to protect.
- River meanders and braids naturally move over time and space.
- Erosion of stream banks is often enhanced at the outside of the meander bend - both outward and downward. Vertical, sandy embankments at meander bends are often evidence of continual undercutting.
- Vegetation and woody debris in the channel and on stream banks can slow river erosion.
- When we place fixed objects and structures in a river's path, we may create scour and damaging erosion, either beside or underneath the structures. This can undermine our own structures, as well as habitat connections for aquatic organisms.