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Abundance and dimensions of naturally occurring macropores along stream channels and the effects of artificially constructed large macropores on transient storage

Macropores are connected void spaces in the subsurface and can act as preferential flow paths for groundwater transport, but their dimensions and distribution patterns have not been well characterized in stream banks and hyporheic sediments. We ran field surveys in 5 streams of varying size, bed slope, and watershed land use in the Appalachian Mountains of southwestern Virginia. Natural surface-connected macropores were nearly ubiquitous. Macropores in this region arise by a variety of mechanisms, including soil piping, tree root decay, erosion around hardened structures, macroinvertebrate burrows, and invertebrate burrows. Macropore openings were slightly wider than tall, with median cross-sectional widths and heights of 3.5 cm and 3.0 cm, respectively. The median and maximum macropore lengths to first bend were 15.0 cm and 120.5 cm into the bank, respectively. True (tortuous) macropore lengths probably are often greater, and methods to map macropores are needed. Median interspacing of macropores across all streams was 0.38 m, but mean interspacing was 1.12 m, indicating that macropores were clustered in space. Macropores were inundated at different times because of differences in their heights on the stream bank, channel geometry, and stream stage. HEC-RAS modeling of channel hydraulics indicated that only 1 to 32% of macropores were inundated by channel water at base flow, whereas up to 97% were inundated during the largest storms. We ran conservative-tracer injection experiments at base flow in a 30-m reach of a small tributary to 1 study stream. We calculated transient-storage parameters for 2 treatments: 1 without macropores and 1 with artificially created macropores. Our results suggest that constructed macropores weakly increased transient storage (channel:storage zone cross-sectional area [As/A] by 15.0% and fraction of median travel time spent in the transient-storage zone [Fmed] by 31.0%) and may affect surface-water–groundwater interactions. Many macropore openings were above typical baseflow water levels, so macropores may enhance bank storage, a possibility that bears further research.