Ancient depositional rivers in arid regions (now expressed as ridges) host important repositories of past surface processes that can be used to infer past climates that sculpted the surface of both Earth and Mars. However, how frequent, how sustained, and under what climatic regimes these systems survived is hindered by the lack of field and laboratory observations that can help place quantitative constraints on their formation and preservation through time. Addressing these fundamental questions may improve our understanding of past climates and aid in the hunt for ancient habitability in these regions on Earth and Mars. Herein, I present five related attempts to quantify the formation and preservation of these ancient rivers through time via observations, measurements, and laboratory analyses. I first compiled new observations as well as those of previous studies; I concluded that, despite the assertation that fluvial ridges record long-lived fluvial activity, there is equivocal evidence for this idea in many fossil rivers (Chapter 2). Indeed, seven sites in Egypt have shown that these ridges likely record both sustained fluvial activity that led to the construction of large-scale paleo-drainage systems and snapshots of fluvial activity due to climatic oscillations in the late Quaternary (Chapter 3). Field observations from southern Egypt, coupled with carbon-14 and optically stimulated luminescence (OSL) dating, lend support to the conclusions that these fossil rivers formed due to high precipitation rates (55–80 mm/hr) and were more frequently active during the African Humid Period, rendering the area inhospitable for human settlements (Chapter 4). These terrestrial observations imply that martian fluvial ridge systems that record single-thread, short-distance source-to-sink courses may have formed due to transient drainage networks that were active over short durations of 10s to 100s of years over a few-thousand-year period (Chapter 5). A diverse array of fluvial systems within the Antoniadi crater on Mars may even record fluvial activity that lasted at least 103–106 years. These events occurred periodically under arid climates, thereby bolstering the hypothesis that episodic warming likely punctuated the planet's early history (Chapter 6). Collectively, these findings place new quantitative constraints on the hydrology and climate of past Earth and Mars; these insights are critical for evaluating their habitability over space and time.