As water demand drastically increased due to growing population and urbanization over the last century, a vast installation of reservoirs proliferated worldwide, fundamentally changing the water cycle (Ripl, 2003). Large-scale water regulation and conveyance systems currently determine present and future water availability to society (Sivapalan et al., 2003, Vogel et al., 2015Vörösmarty & Sahagian, 2000). In the United States (U.S.) alone, more than 90,000 dams change the quantity and variability of natural flow regimes, altering an estimated >85% of inland waterways (National Research Council, 1992). The impacts of such alteration propagate through river networks and affect the fluvial ecosystem in multiple ways: by preventing sediment transport (

Dams are a major contributor to flow regime alteration: they increase water residence time, mute peak flows, shift the timing of ecologically important high and low flows, and alter flow periodicity (Poff et al., 2007;Ruhi et al., 2018;Vorosmarty, 1997). These alterations adversely affect riverine and riparian biodiversity because the life-history, morphological, and behavioral adaptations of organisms are often at odds with the novel environmental regime (Bunn & Arthington, 2002;Lytle & Poff, 2004;Mims & Olden, 2013). Such flow regime alteration is also often detrimental to society, as it may disrupt floodplain fishing, flood-recession agriculture, and a wide range of recreational and cultural values (Anderson et al., 2019). Despite intense scrutiny, flow alteration by dams has largely been estimated via methods that do not take into account the spatial context in which these changes

The natural flow regime of a river is defined by the characteristic pattern of the streamflow variability (Poff et al., 1997). The natural flow regime impacts the riverine habitat, biodiversity, water quality, and overall river ecosystem health (Poff et al., 1997). The alteration of the flow regime is a major factor in degrading the river ecosystem (Poff et al., 2010). The flow regime may change due to human intervention through dams, barrages, land-use and land-cover changes, and changes in the region's hydroclimatology. The world's rivers are regulated through an estimated 2.8 million dams (Lehner et al., 2011) for meeting various human needs such as municipal water supply, flood control, irrigation water supply, and recreational uses. Such regulations alter streamflow in various time scales-hydropeaking alters flow at hourly time scales whereas low flow augmentation affects summer flows, and inter-basin transfers impact the flows over long-term (annual and beyond) (Alonso et al., 2017;Bunn & Arthington, 2002;Stanford & Ward, 1979). The various kinds of flow regulation introduced by these dams have adverse ecological consequences (Poff et al., 1997) as they fragment aquatic habitats (Nilsson et al., 2005) and hinder the movement of species, nutrients, and sediments along the stream (Lehner et al., 2011). Furthermore, such alteration affects the river regime in multiple forms by propagating through river networks multiple ways such as by destabilizing channel morphology (Graf, 2006), by altering the composition and dynamics of aquatic biota (Poff et al., 2007), and by reducing sediment transport (Lehner et al., 2011).Reservoir regulation has been studied over many decades (

Sufficient understanding of the hydrological processes and catchment controls is essential for predicting water availability and enhancing water resources reliability for human society and ecology. In this regard, several critical studies have shown that the intricate interactions between climatic forcings (precipitation and temperature/ potential evapotranspiration) and catchment characteristics are the dominant controls for the water balance of a catchment (Eagleson, 1978;Farmer et al., 2003;Milly, 1994;Zhou et al., 2015). Accordingly, hydroclimatic models are often adopted to simplify the complex hydroclimatological process by selectively amplifying a system's fundamental aspects at the expense of incidental details. Thus, a model is considered ideal when it is simple enough to understand and use while complex enough to reflect the hydrological process (Anderson & Burt, 1985). To this end, numerous hydrological models have been developed along with the advances in hydrology, data collection, and computational capability (

In many cases, persistent or recurrent synoptic circulation patterns lead to multiple wet days that precede an extreme rainfall event. The joint occurrence of high antecedent rainfall and extreme rainfall defines a compound event that may pose a high risk for overtopping of aging dams. Our novel analysis assesses whether there are significant trends across the conterminous United States (CONUS) in the joint and individual occurrence of extreme daily precipitation and k-day antecedent precipitation extremes, for k=5 and k=30 days. We find significant trends in the mean and variance of annual maximum daily rainfall, and in the k-day antecedent precipitation in certain regions of the CONUS. However, their mutual dependence as measured through a copula is invariant with time. The probability of their joint exceedance, i.e., simultaneously experiencing high extremes of daily precipitation and the k-day precipitation total, is also increasing in many places in the CONUS.