A warm, dry autumn has made for great foliage, but there are growing concerns about the impact of a fall with no rain. Fall of 2024 has been historically dry, with Philadelphia having the longest dry period since weather record-keeping began 153 years ago – an unbelievable 42 days with no rain. For the first time in the region, there was no rain in October, and temperatures were unseasonably warm, which can cause more water to leave the soil. While the dry period ended with measurable rain on November 3 - 0.31 inches of rain was recorded at the Philadelphia Airport - Chester and Delaware Counties are still facing drought conditions.
Unsurprisingly, the lack of rain has a significant impact on the health of local waterways. As rain continues to be elusive in the region, the amount of flowing water in many waterways continues to drop causing physical changes to stream habitat. Low flow conditions can cause dramatic changes to resource availability: the resulting changes to the flow patterns and currents can cause sections of normally flowing streams to become isolated and turn into pools, trapping wildlife in stagnant water. Decreased water levels also means that there is just less space for wildlife in the stream, causing crowding and more competition for fewer resources. In some cases, small streams can dry up entirely. This dramatic loss of habitat has been documented in the Darby Creek Watershed already during this historic drought, with many volunteers reporting that the water level at their site is much lower than usual and one site, DCWP1, has been reported as completely dry since September.
Dry streambed at DCCCS Site DCWP1. Photo courtesy of Ivan Burashnikov.
Droughts also cause changes in water chemistry: changes in flow patterns can influence specific conductivity (SPC or just conductivity) and chloride levels. In periods of drought, conductivity can increase in stream systems and is often a reflection of chloride concentration. Conductivity measures the water's ability to carry an electrical current, which depends on the concentration of dissolved ions like salts, minerals, and other charged particles. High conductivity levels can indicate pollution from sources such as road salt, sewage, or agricultural runoff.
While some chloride is present in the geology of Chester and Delaware Counties, the majority of chloride is introduced into local watersheds through the application of road salts (usually sodium chloride, NaCl) to melt snow and ice in the winter. As the snow and ice melt, the salt flushes into local waterways. In areas where salt is applied frequently and in abundance, the chloride can build up in the soils, leading to salty groundwater and high levels of chloride year round – instead of just after winter storms. Chloride, including the form road salt, is known to be harmful to sensitive fish and invertebrates in freshwater systems. When salt buildup in soils meets drought conditions, it leads to salty streams. Less rainfall, (or in the case of October 2024 no rainfall), leads to little to no dilution of groundwater entering the stream, increasing the concentration of chloride in local waterways. When these dry conditions are paired with unseasonably warm temperatures, it is a recipe for rapid evaporation of surface waters. Evaporation leaves chloride behind, causing even higher concentrations of chloride ions. During a severe drought, stream systems are almost entirely fed by groundwater, which means the potential for higher concentrations of chloride ions. In the DCCCS protocol, conductivity and chloride measurements are taken together, which can indicate if high conductivity is because of chloride or from another source.
This pattern became evident when looking at the DCCCS data collected during this historic dry period: when comparing conductivity (SPC) data from the November sample period, at the height of the drought, to average values, there are 18 sites with above average SPC, two sites with below average SPC, and five sites with average SPC (Figure 1). A similar trend occurs when making the same comparison with the chloride concentration data: 16 sites with above average chloride concentrations, 2 sites with below average chloride concentrations, and 7 sites with average chloride concentrations (Figure 2).
As the graphs demonstrate, the lack of rain has played a large role in the future health and function of Darby and Cobbs Creeks. When these trends are laid out on the landscape of the Darby & Cobbs watershed, it becomes apparent that there is a complicated relationship between the land, water, and the weather (Map 1). Some sites have seen no notable change or even a decrease in conductivity during the drought (DCYM1, DCCM1, DCSW1), while other small streams (DCCL1, DCIC1, DCLR1) had an increase in conductivity. Reasons for this are complicated - these sites may have less salt contamination in groundwater sources or there may be more surface flow.
Interactions between surface water and groundwater are complex and remain poorly understood across the entirety of the watershed. Many of the sites most highly impacted by the drought are small tributary streams, while the Cobbs Creek remain fairly stable with fairly consistent elevations in SPC (CCMP1, CCPO2, CCPO1, CCNR1, CCNR3, CCEC1). However, as the Darby & Cobbs Creeks flow towards the Delaware River, the streams become larger: more water flowing in the channel often causes lower chloride and conductivity readings, which makes the increases in SPC across the entire lower Darby Creek notable (DCHF2, DCMP1, DCRG1, DCHP1, DCSC1, DCFO1).
Long-term, high-frequency water quality data, such as those you collect, are invaluable for understanding the impacts of these unusual weather patterns on stream ecosystems. It takes consistent data collection to paint a detailed picture of the health and function of a river system. These are preliminary results that we will continue to explore as more time, data, and droughts are added to the data set.
Interested in learning more?
USGS Pennsylvania Drought Map: https://rconnect.usgs.gov/PA_drought_map/
NOAA Pennsylvania Drought Map: https://www.drought.gov/states/Pennsylvania
Check out these USGS stations to view discharge, depth, temperature, and conductivity data in real time:
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