An estuary is a enclosed coastal body of brackish water with one or more rivers or streams flowing into it, with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments known as ecotone. Estuaries are subject both to marine influences such as tides and the influx of saline water and to riverine influences such as flows of freshwater and sediment; the mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are classified according to their geomorphological features or to water-circulation patterns, they can have many different names, such as bays, lagoons, inlets, or sounds, although some of these water bodies do not meet the above definition of an estuary and could be saline.

Many estuaries suffer degeneration from a variety of factors including soil erosion, overgrazing and the filling of wetlands. Eutrophication may lead to excessive nutrients from animal wastes; the word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary; the most accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, within which seawater is measurably diluted with freshwater derived from land drainage". However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas. A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; this broad definition includes fjords, river mouths, tidal creeks. An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides.

The seawater entering the estuary streams. The pattern of dilution varies between different estuaries and depends on the volume of freshwater, the tidal range, the extent of evaporation of the water in the estuary. Drowned river valleys are known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley; this is the most common type of estuary in temperate climates. Well-studied estuaries include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border; the width-to-depth ratio of these estuaries is large, appearing wedge-shaped in the inner part and broadening and deepening seaward. Water depths exceed 30 m. Examples of this type of estuary in the U. S. are the Hudson River, Chesapeake Bay, Delaware Bay along the Mid-Atlantic coast, Galveston Bay and Tampa Bay along the Gulf Coast. Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels so that the estuaries are shallow and separated from the sea by sand spits or barrier islands.

They are common in tropical and subtropical locations. These estuaries are semi-isolated from ocean waters by barrier beaches. Formation of barrier beaches encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries develop on sloping plains located along tectonically stable edges of continents and marginal sea coasts, they are extensive along the Atlantic and Gulf coasts of the U. S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 m. The barrier beaches that enclose bar-built estuaries have been developed in several ways: building up of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline, reworking of sediment discharge from rivers by a wave and wind action into beaches, overwash flats, dunes, engulfment of mainland beach ridges due to sea-level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, elongation of barrier spits from the erosion of headlands due to the action of longshore currents, with the spits growing in the direction of the littoral drift.

Fjords were formed where Pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are rocks, bars or sills of glacial deposits, which have the effects of modifying the estuarine circulation. Fjord-type estuaries are formed in eroded valleys formed by glaciers; these U-shaped estuaries have steep sides, rock bottoms, underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m

Pondosa is an unincorporated community and ghost town in Union County, United States. Pondosa has an elevation of 3261 feet; the town came into being in 1927, when the four Stoddard brothers of La Grande bought land in the area. They moved the sawmill operations of the Grande Ronde Lumber Company in nearby Perry to the site; the town was named after the Ponderosa Pines. A post office was established in 1927. In 1931, Truman Collins of The Collins Companies of Portland bought the mill; the mill was sold again in this time to Boise Cascade, which made plans to shut down the mill. This caused the decline of the town; the post office was discontinued following a severe fire that damaged the majority of the town on June 20, 1959

The Battle of Xiangyang was fought between the warlords Sun Jian and Liu Biao in 191 in the late Eastern Han Dynasty. Liu Biao emerged victorious against Sun Jian's forces. Shortly after their coalition had ousted Dong Zhuo from the capital Luoyang, Yuan Shu and Yuan Shao, two feudal lords vying for power, had formed alliances against one another, with Gongsun Zan supporting Yuan Shu while Liu Biao supported Yuan Shao. Yuan Shu sent his subordinate Sun Jian to attack Liu Biao in order to extinguish Yuan Shao's influence in the southern half of China. Although Sun Jian outmaneuvered and outfought Liu Biao, he was killed in action and his army forced to retreat. Sun Jian and his forces encountered the forces of Liu Biao's general, Huang Zu, between Fancheng and Deng. There, he routed Huang's forces and surrounded the city of Xiangyang itself. Within the confines of the city, Liu Biao again sent Huang Zu out, this time to make a surprise attack. However, Huang was again defeated, when he attempted to withdraw to the city once more, Sun Jian cut off his line of retreat, he fled to Mount Xian.

Sun pursued Huang. According to Sun Jian's official biography in Records of Three Kingdoms, he was fatally wounded by an arrow fired by a soldier in Huang Zu's unit, hiding in a bamboo grove. Sun's men carried him away from the battlefield and he died from his injury later. Other theories of Sun Jian's death include Sun being killed while fighting in the wilderness, or being crushed to death by boulders rolled down by the enemy from above. Sun Jian's death ended the battle, although Liu's forces suffered far more casualties. Huan Jie, an official under Sun Jian negotiated for his lord's corpse to be returned, Sun Jian's army temporarily came under the control of Sun Jian's nephew, Sun Ben. Sun Ben went to join Yuan Shu with most of Sun Jian's followers. Sun Ce, Sun Jian's oldest son, was given Sun Jian's marquisate, but chose to pass the position down to his youngest brother, Sun Kuang, still young then. Liu Biao's success allowed him to expand his influence in Jing Province. Though he would be unable to defeat either Sun Ce or his successor, Sun Quan, he would nonetheless remain an influential figure in the Han dynasty until his death in 208.

The battle and its justifications were altered in the 14th-century historical novel Romance of the Three Kingdoms. In this fictionalized account, Sun Jian attacked Liu Biao for revenge because Liu's forces routed his army during his withdrawal from the coalition against Dong Zhuo. Sun Jian's younger brother, Sun Jing, attempted to dissuade him from this course of action, but Sun Jian refused to listen. Sun Jian's oldest son, Sun Ce, was granted a command position and performed well in his first battle. After defeating Huang Zu, Sun Jian's army surrounded Xiangyang. Two commanders under Liu Biao, Chen Sheng and Zhang Hu, were killed by Sun Jian and Han Dang, respectively. Sun Jian was killed in a rockslide after being lured into a trap by Lü Gong and Huang Zu, according to a ploy by Liu Biao's advisor Kuai Liang. Sun Jian's generals Huang Gai and Cheng Pu managed to capture Huang Zu and kill Lü Gong while Sun Ce was forced to withdraw. Huan Jie's involvement in recovering Sun Jian's corpse is similar in the novel to the historical account.

However, in the novel, Huang Zu was released in exchange for Sun Jian's corpse, when there is no mention of Huang Zu's capture in historical texts. The Battle of Xiangyang is featured in video games such as Koei's Dynasty Warriors, but the name of the battle changes with every release. After Sun Jian is killed, Sun Ce takes command of the army instead of Sun Ben. Sun Jian's death changes with each game release, as a possible reflection of the confusion as to his actual cause of death