The Delaware and Raritan Canal is a source of drinking water for 1.5 million people in eastern New Jersey. The Canal is fed by the Delaware River upstream of Stockton, and by the Lockatong and Wickecheoke Creeks. (Forty percent of the land area of these two watersheds lies in Delaware Township, with the remainder in Franklin, Kingwood and Raritan.) A study by the Natural Resources Conservation Service (NRCS) of the U.S. Dept. of Agriculture has found that in the last decade the amount of sediment entering the Canal from the Lockatong and Wickecheoke Watersheds has increased dramatically. The New Jersey Water Supply Authority (NJWSA) manages the water in the Canal and in the last decade its cost for sediment removal has risen to $3 million dollars a year. To reduce sedimentation in the Canal, the NJWSA and NRCS are developing a Lockatong and Wickecheoke Watershed Protection and Restoration Plan. A steering committee of concerned stakeholders from the two watersheds, residents of Delaware, Franklin, Kingwood and Raritan Townships, met at Prallsville Mills on June 21 to hear the results of a study on land use and erosion in the watersheds.
Forest covers about 31 percent of the land area in the two watersheds. A 2006 study found little erosion in woodlands, and stated that the limited forest activity should not cause excess sediment loss. Estimates are that about 355 tons of sediment enter the D&R Canal annually from woodlands.
Construction sites represent a very small fraction of overall land use. The NRCS study notes that “there is minimal delivery of sediment from construction sites due to the relative lack of connection to the existing natural drainage network”. Active construction sites within the watershed “do not have a significant impact on sediment production to the D&R Canal”. The NRCS interim report does not, however, address the cumulative effect of new housing development in recent years.
Farmland covers 40 percent of the watersheds, according to a study done in May 2007. NRCS identified over 1,500 crop fields; its method for determining soil loss involved random sampling of twenty percent of these fields. NRCS recognizes that the study thus far has relied heavily on modeling. “A gross projected soil loss of 11,978 tons per year results when the average erosion rates are projected for all the cropland”.
The NJWSA is doing water quality and flow sampling and some tromping around in the fields. They have learned that there are hidden cases of erosion, such as gullies in the tree rows between fields, and ditches forming at the ends of old terraces built by the SCS/NRCS, that aren’t part of the models. Small erosion ditches become large ones, and the NJWSA/NRCS are aware of the importance of stopping this process. This has given rise to a new jargon term “micro topographic management”, which basically means repairing a drainage issue with a wheelbarrow and shovel while you still can, before you need a truck and tractor.
In general, though, the models indicate that the trend away from row crops, corn and beans in favor of permanent hay should lead to reduced soil loss and runoff. It is assumed that sediment can take decades to wash all the way downstream, say from Croton to Stockton. This “legacy sediment”, which gets picked up and carried further downstream during heavy rains and snowmelts, is considered to still be a cause of streambank erosion.
Streambank erosion is recognized to be a significant source of sediment delivery to the Canal. Depending on the method used, estimates range from 248 to 12,295 tons of soil loss per year. More work is needed to get a better understanding of this.
Roads cover 337 acres, or approximately 1 percent of the watersheds, according to the May 2007 report. However, the study shows that roads contribute more sediment per area of coverage than all other land uses. “An analysis, using GIS technology, of the stream drainage network vs the effective drainage network (includes the road network) shows that the effective drainage network of the watersheds and their subwatersheds is increased significantly, up to 75%, by the road network. Field observations of paved and unpaved roads in these watersheds indicate that there can be a disproportionate (in terms of the land area affected) amount of sediment moving from both the roads surface, graded and mowed right of way, and drainage pipes and attendant unprotected (from erosion) outlets. Also, where road stream crossings occur, there is uncontrolled road runoff entering the stream without any sort of pre-treatment. Frequently there is little, if any, area for the installation of a detention basin, check dams or other structural measures due to the steep slopes and relatively narrow stream corridors. A total of 3,723 tons per year of soil loss is estimated to occur from paved and unpaved road surfaces exclusive of any road treatment (such as sand, etc.) used for deicing”.
There are 350 miles of natural stream in the Lockatong and Wickecheoke Watersheds, and 139 miles of roads. The NJWSA and NRCS have observed that the road network functions as an extension of the natural stream network, and have coined the term “effective drainage network” to describe the two networks together.
Road surfaces are smooth impervious cover; the rights of way are usually graded smooth and mowed; drainage ditches are dug out every year or two and left without vegetative cover. The County has an ongoing road widening program. Each year more roadside drainpipe is installed, increasing the volume and velocity of runoff into the streams. Due to all these management practices, the road network puts water into the stream network faster than the natural tributaries do. Increased volume is recognized as a direct cause of streambank erosion.
The study notes that the erosion of streambanks is starting to undermine roads. Repairs to Lower Creek Road will cost Delaware Township over $100,000 this year alone, with more washouts anticipated.
The emerging issue is volume of stormwater runoff. The streambanks are eroding because there are unprecedented levels of it. Farmland is generating less because farmers are switching to hay, and forests generate very little. Houses and roads are generating more of it, and attention is starting to focus on these two sources. Land use decisions on the thousands of housing lots in the watersheds are left almost entirely to the owners. Management decisions on Township and County roads, though, lie with the Road Departments, which are at least nominally under the control of the Township and County governments.
The NRCS interim report is part of the ongoing Lockatong and Wickecheoke Watershed Protection and Restoration Plan. A full report with recommended land management practices for forest, agriculture, housing and roads will be issued next year.