Abstract: Diurnal streamflow (or water level) cycle is an important indicator of hydrologic regime. Analyzing the diurnal variation of streamflow (water level) can reveal the source of runoff and its formation mechanism, and clarify the hydrological processes/dynamics of the river. The reaches of six important tributaries (namely, Yigong Zangbo, Parlung Zangbo, Quzong Zangbo, Layuequ, Jinzhuqu, and Baima Xilu rivers) in the lower Yarlung Zangbo River located in Southeast Xizang, were collected to investigate the spatiotemporal characteristics of the daily water level changes. Based on the monitored hourly-resolution water level data of the six river reaches during 2022—2023, the diurnal water level characteristics in the study area were studied using mathematical statistics and parameter analyses. Factors (including rainfall, glacier, and snow melting) influencing the diurnal variation of water level were also investigated. The diurnal variation of water level during flood season is mostly higher than that during non-flood season; the distribution of daily water level is mostly positively skewed during the flood season (mean value higher than median value) and is negatively skewed during post-flood season (mean value lower than median value). Except for the Baima Xilu River, where the daily water level changes have no marked seasonal difference, the start time of daily water level rise/fall during non-flood season is normally later than that during flood season in each analyzed river reach; this delay is more pronounced in the post-flood period. The occurrence time of daily maximum and minimum water level in each reach is more centrally distributed during the flood season, and usually earlier than that during the non-flood season. The duration of daily water level rising (i.e., the time span for the water level to go from the daily lowest to highest) is shorter than falling (from the daily highest to lowest) during flood season in each reach; however, the trend is almost opposite in non-flood season except for the Layuequ and Jinzhuqu rivers. The diurnal variation of water level of the Baima Xilu River is mainly disturbed by rainfall, while that of other rivers is primarily influenced by snow/ice melting.