RARITAN RIVER
The Raritan River is a major river of New Jersey. Its watershed drains much of the mountainous area of the central part of the state, emptying into the Raritan Bay on the Atlantic Ocean. The river forms at the confluence of the North and South Branches just west of Somerville at the border of Bridgewater, Branchburg, and Hillsborough Townships. It flows for approximately 16 miles before slowing in tidewater at New Brunswick, and its estuary extends 14 miles more entering the western end of Raritan Bay at South Amboy.
Why Does The River Flood & What Is The Result?
The Raritan River has persistent flooding problems when excessive rain from storms affects the river basin. The flooding problems mainly affect the town of Bound Brook, which is partially built on a natural floodplain at the junction of several tributaries, and Manville, which has a large neighborhood known as Lost Valley that lies on the floodplain between the Raritan River and its largest tributary river, which is known as the Millstone River. Other towns in the Raritan River basin also experience flooding to a lesser degree. It is one of the major flooding rivers in the state.
A flood occurs when a river or stream overflows its banks. Seasonal floods are the norm in many rivers, for example when spring rains or snowmelt increase the flow. During a flood, the channel is completely filled and water moves onto the floodplain and slows down. As it slows, it can carry less material. A lot of sand, silt, leaves, and other materials that were carried while the water was swirling along in the channel are dropped on the floodplain.
A natural river channel is shaped by the amount of water and sediment that travels through it. Even though rivers can vary greatly in their form, a natural river’s channel is almost always sized to carry the largest amount of water that flows through the system once every two years. Another way to put this is to say that rivers in their natural state are designed to flood approximately half of all years.
Constant rising water levels and past flooding leave their mark along the banks of a river. In some instances you can see undercutting of the bank toe as a sign of scour and mass failure. Mass Failure: Characterized by sections of the bank sliding or toppling into the stream. Look for bare and near-vertical banks as a sign of collapse. Exposed tree roots and cracks in the soil at a river bank are also signs of bank erosion. There are many other signs of bank erosion but these are the most common.
River bank erosion makes a river unstable. The four major categories that suggest a river is unstable are as follows:
1. Degradation: The river cuts deeper, down into the earth. The undermining and exposure of bridge footings caused by river action, for example, is often the result of degradation. Degradation is sometimes caused by straightening a channel, which can increase the slope of the river. Water flows faster down this steeper slope and has extra energy to move sediment, causing the river channel to cut deeper or degrade.
2. Widening: Banks collapse, and the river becomes wider and shallower. Bank collapse and widening follow river degradation, but may also occur when there is increased stormwater runoff. A wider, shallower river does not have the same capacity to move sediment, so soil can build up in the channel.
3. Aggradation: Sediment loads pile up in the river. This happens when the amount of sediment in the river water increases, and the river lacks the capacity to carry it downstream. In a typical river evolution process, aggradation typically follows channel widening. Piles of sediment in a river can re-direct flow against the banks, causing yet more streambank erosion.
4. Changes in “Planform”: These are the changes that often follow excessive aggradation and can be seen from the air when looking down at the river. Planform changes can occur during floods. When there is no streambank vegetation with roots to hold soil in place, rivers cut new channels in the weak part of the bank during high water.
QUESTIONS/TASKS:
1. What is leading to the erosion in this area?
2. What signs of erosion are along the banks of the river at this location?
3. Can you see any bedrock or uprooted coastal grass at this location due to the erosion?
4. To prove that you were here, please post a picture of yourself or your GPS with the river in the background. Any other photos of the area are also greatly appreciated.