Simple measures of channel habitat complexity predict transient hydraulic storage in streams

Stream thalweg depth profiles (along path of greatest channel depth) and woody debris tallies have recently become components of routine field procedures for quantifying physical habitat in national stream monitoring efforts. Mean residual depth, standard deviation of thalweg depth, and large woody debris (LWD) volumes are potential metrics of habitat complexity calculated from these survey data. We used 42 intensive dye-transit studies to demonstrate the relevance of these easily measured channel habitat complexity metrics to transient hydraulic (dead zone) storage, a channel process important for biotic habitat as well as retention and spiraling of dissolved and particulate nutrients. We examined transient storage and channel morphology in small gravel and cobble-bedded upland streams (wetted width 2-5 m; slopes 2.6-8.3%) representing a wide range of flow stages, LWD loading, and channel complexity, including measurements before and after LWD was added to enhance fish habitat. While transient storage volume fraction decreased as flow stage increased in simple channels, those with complex morphology and well-developed riparian vegetation maintained high transient storage fractions even during storm flows. LWD additions increased transient storage and channel complexity over the 2 years of post-treatment measurements. We predict with considerable precision two different formulations of transient hydraulic storage fraction using single-variable linear regressions on residual depth (R (2) = 0.61-0.89), thalweg depth variance (R (2) = 0.64-0.91), or large woody debris volume (R (2) = 0.48-0.74). Demonstration of these likely causal associations contributes to understanding the process of transient storage and redefines the use of thalweg profile metrics as a new approach to quantifying morphologic and hydraulic complexity in streams.