Final answer:
A landform that obstructs water flow into a lagoon is typically a sand spit or barrier island, formed by deposition of materials carried by the river. Man-made structures like weirs or dams can also obstruct flow for various purposes.
Step-by-step explanation:
The landform that partially obstructs water flow into a lagoon near the mouth of a river, such as the one near Tweed River, is typically a barrier such as a sand spit or a barrier island. These formations are often triangular-shaped and formed from the deposition of sand and silt carried by the river's flow. As the water slows down, it deposits these materials, which can obstruct the flow into the lagoon, creating an area of relatively shallow water. This type of landform separates the lagoon from the open marine conditions but typically allows some water to pass through narrow inlets.
In some cases, man-made structures like a weir or dam can also act to obstruct water flow in coastal areas. The presence of such structures can control the movement of water for purposes like preventing floods, generating hydroelectric power, or creating stagnant conditions with fine sediment deposition.