This paper demonstrates that the multivariate monitoring methods are capable to underpin the systematic investigation of the hysteretic behavior occurring during gradually-varied flows. For this purpose, we present simultaneous measurements of stage, index velocity, and free-surface slope acquired continuously with high-frequency sampling instruments deployed at several river gauging sites exposed to a range of storm magnitudes. The experimental evidence reveals intrinsic features of unsteady open-channel flow mechanics that are hinted by pertinent governing equations but rarely substantiated with in-situ measurements. The illustrations are intentionally made for fluvial waves propagating at sites located in lowland areas where the relationships among flow variables are most likely displaying hysteretic loops and phasing in the hydraulic variable progression. The set of presented measurements highlights that: a) the hysteretic behavior is apparent in both time-independent and time-dependent graphical representations of any two of the hydraulic variables; b) the severity of the hysteresis is commensurate with the geomorphic, hydraulic, and hydrological characteristics of the measurement site; and c) there is a pressing need for changing the flow paradigms currently used in tracking flow variables during gradually-varied flows. Also discussed are research needs associated with flow hysteresis for advancing the understanding of the mechanisms underlying the movement and storage of water in the lowland river environments as well as for increasing the accuracy of streamflow monitoring, modeling, and forecasting.

Advancing the understanding of watershed dynamics and underpinning scientific studies on the changes in water cycles, ecological patterns, and climate trends often rely on streamflow data acquired at gaging stations operated by various monitoring agencies. The monitoring methods used at these stations are based on empirical or semi-empirical rating curves obtained with statistical analysis uniformly applied to datasets collected in steady and unsteady flows. These one-to-one ratings are subsequently used for estimating steady and unsteady flows, even though, in the latter case, the relationships between flow variables are different for the rising and falling phases of the flow hydrographs. The non-singled relationships are more prominent in lowland areas subjected to high flows, where the streamflow variables display a hysteretic behavior. Recent advances in measurement technologies (e.g., acoustic-, radar-, and image-based), have dramatically transformed our capabilities to conduct in-situ measurements. This paper demonstrates such capabilities by presenting new information extracted from directly measured data and novel algorithms applied to simultaneous measurements of stages and index velocities acquired with a SonTek Side-Looker (pertaining to the family of Horizontal Acoustic Current Profilers-HADCP) at a river location prone to hysteresis. The presented results demonstrate a) the capability of the conventional monitoring methods (i.e., index-velocity approach supported by HADCPs) to capture the dynamics of the unsteady flows, b) the opportunity offered by HADCP measurements to re-think monitoring methods altogether by using directly measured data and their spatial and temporal gradients in conjunction with canonical flow equations (i.e., St Venant Equations) without using ratings, and c) the opportunity to exploit subtle features of the hysteretic behavior for developing short-term forecasting of flood crest magnitude and its arrival time using only in-situ acquired data, without making recourse to hydrologic/hydraulic modeling. Moving away from the traditional empirically based ratings, would unquestionably contribute to reducing uncertainties related to modeling flow routing, thus improving the quality of conventional forecasts.