Impacts of Tiling on Hydrology
There are a variety of effects to the hydrology of the SMC Watershed that can be attributed to tile drainage. The most prominent influence is that tiling provides an expressway by which water on the fields is drained away and discharged into a nearby open ditch, which is flowing rapidly into Seven Mile Creek proper.
Ditch vs. Stream Mechanics
The above picture is of an open drainage ditch running through cultivated land. The tile lines below the surface of the fields on either side of this ditch carry water to the ditch, and the ditch, in turn, carries that water to Seven Mile Creek. The pubic ditches running throughout the watershed are all trapezoidal earthen canals, like this one. There are two main structural features that separate a ditch from a natural, free-flowing stream. Firstly, ditches are confined to a straight channel, while streams naturally meander and bend. Secondly, ditches have smooth, compact earthen banks and bottom, but streams are loaded with cobbles, gravels, loosely compacted sediments and organic material. Hydrologically, the implication of ditch design as compared to that of a natural stream is faster moving water (higher flow velocities) and, ultimately, greater potential for erosion. For a more detailed explanation of the relationship between stream structure and flow velocity click here.
As flow velocity in a stream increases, the flow will tend to carve a deeper channel in an attempt to reach a new equilibrium with the streambed. A faster moving stream is able to carry more and larger sediment, so it, effectively, lifts up and carries away the top layer of the streambed. Another implication of higher flow velocities is the increased energy associated with faster flowing water. When confronted with a meander, streams with faster flows have the ability to cut deeper into the bank and erode more. This is an issue of particular concern in SMC Watershed, because the natural channel of Seven Mile Creek is fed by open ditch systems at two major inputs. At these input points, flows of high velocity are entering a channel that is naturally in equilibrium with slower flows. Therefore, the banks in the section of natural stream are highly susceptible to erosion on a large scale, as seen in the picture below.
The banks of SMC are highly susceptible to the erosive forces of fast moving water ruching out of the public ditch system.
Creating a Flashy Hydrograph
A second hydrologic effect of tiling is observed in the hydrograph of Seven Mile Creek. By providing an expressway from the fields to the creek, tiling allows precipitation to bypass the natural residence time associated with absorption of rainwater into soils and seepage through those soils, which is controlled by the hydraulic conductivity of the soil. Designed to protect crops from being drowned, tile lines remove any water from the fields that is not soaked up by plants and dump it into open ditches.
Tile line discharging into open ditch.
This influx of water makes its way through the ditches, gaining speed with the added volume of water introduced at each tile input, until it is forced through a culvert and into Seven Mile Creek. Here are two pictures that illustrate the severity of "flashiness" observed in Seven Mile. The picture on the left was taken in April of 2000, during a time of normal to low flows, and the picture on the right was taken during a storm event in the same month one year later.
The large eroded bank and minimal flow of water seen in the picture on the left is evidence of a stream that is not in equilibrium with its channel. This is the result of the increased energy associated with high flow velocities as they encounter a stream bank used to slower waters. On the right, the creek appears dirty from the heavy load of eroded sediment it is carrying downstream
Cutting Off Recharge
The final hydrologic impact of tiling relates to recharge of local aquifers. Though further investigation into the hydrogeologic setting of SMC Watershed would be necessary to determine the actual connectivity of the upland recharge zone with the Jordan aquifer directly below it, it is worth addressing the issue as a potential reality. Tiles are installed 3 to 4 feet below the land surface and usually spaced 12 to 15 feet apart. As noted previously, there may be as much as 75 miles of tile underlying the SMC Watershed. Such an extensive drainage net has the ability to collect a lot of water and prevent it from percolating any deeper than 4 feet into the ground. If the upland hydrogeologic unit of loamy soils and underlying till is hydrologically connected to the Jordan sandstone below, then the upland region could serve as a significant zone of recharge for the Jordan aquifer, locally. Due to the presence of the small spring upwelling at the banks of the creek within Seven Mile Creek Park, we know that the Jordan is, in fact, at least partially saturated in the region of SMC Watershed and there is substantial groundwater flow through the unit. (The spring maintains a flow of 1-3 cfs in the lower stretch of the creek when upper portions are completely dried up in winter months.) The central question is where is the source of the spring and how is that unit being recharged? If the upland region is the recharge zone, we can begin to assess the impact that land use and land cover change as well as drainage practices have had on spring flow over time. The primary concern relates to maintaining a viable trout stream, which can only be supported by the spring. Understanding where recharge is coming from may lead to groundwater policies to ensure sufficient spring flows for anglers in the future. This issue is interesting from the perspective of a geologist, as well. How is groundwater moving within SMC Watershed? How is it travelling through the till? What direction does it flow within the Jordan? How has man influenced local hydrology? These are all questions that can only be answered through research, but the confounding hydrogeologic possibilities and associated impacts of historical land use change have been outline here.
In addition to hydrology, water quality in the SMC Watershed has been impacted over time. At the next stop, we will learn about water quality monitoring methods and the contaminants posing the greatest threat to Seven Mile Creek.