Unlike the watersheds of Kâne'ohe and Kailua, the Waimânalo watershed has retained much of its rural character. Waimânalo's water resources were historically diverted from areas in Kailua (e.g., upper Maunawili Valley) for agriculture. Prehistoric burial sites and ancient Hawaiian villages have been recorded near the shoreline, which, along the central portion of Waimânalo Bay, consists of beach sand backed by a series of dunes that are among the most extensive on O'ahu. The reef platform off Waimânalo is well developed although somewhat submerged.

Wailea Point (also known as Popo`oka`ala) is a volcanic headland marking the western end of Waimanalo Bay. Seawalls line the shoreline around the Point, and their presence has probably contributed to loss of sand in several shoreline sections nearby (Clark, 1977). (1)

One of the most visible features along Waimânalo Bay is Waimânalo Beach, which, with an overall length of nearly 5.5 mi (8.8 km), is the longest stretch of sandy shoreline on O`ahu. Waimânalo Beach is a popular sun-bathing and swimming area, especially where its sandy bottom slopes gently offshore. Lithified dunes occur on the alluvial plain behind the beach. Located just north of Waimânalo Beach, Bellows Air Force Station, with its several ditches, streams, and wetland areas, provides habitat for endangered waterbird species. Shoreline access permits moderate to heavy fishing for ulua, pâpio, weke, and 'ô'io along the length of Waimânalo Bay.

Waimanalo Valley, long a sugar plantation and now [1972] a ranch, had less wet taro in olden days, being blessed with only one stream. Yet much of what was until recent years sugar-cane land had previously been planted in taro. There were evidences in 1935 of old lo`i much further inland, in a semicircle at the back of the broad valley. A kama`aina of the place at that time named nine such lo`i sections whose water came from springs. Levi Chamberlain is quoted (Sterling and Summers, 1962, BK 5, Vol. 2, p. 344) as reporting in 1828 the location of a small and quite poor fishing village near the beach, toward Makapu`u Point from the present Waimanalo town, just beyond which there was a pool named Ka-wai-kupanaha where these people got their fresh water. This has since been covered by the roadway. It is probably adjacent to this site that the remains of a fishing shrine (ko`a) are visible on a point of land just offshore, surrounded by water at high tide (McAllister, 1933, p. 195). Beyond the old plantation town of Waimanalo and toward Makapu`u Point is a narrow stretch of land lying between the dry windward face of this southeast end of the Ko`olau range and the sea, the name of which was Ko`o-o-na-pou (mistakingly called Kaupo in recent times). This was a sweetpotato planting area. A village was established here by a kahuna who had a peculiar grass house with two rooms: the front room into which visitors came; and his private room behind this, which abutted on a low cave with a rather thin roof of lava shaped like a flat dome. In this little cave the kahuna kept his paraphernalia. The site was exposed to heavy winds, so the house frame was braced by heavy props (ko`o) that held the posts (na pou) secure against the winds of the sea. The village and the land took their descriptive names from this house. The scattered rocks where the house had been and the little lava dome were carried off during World War II, and likewise the stones of a fisherman's heiau on the rocky foreshore where the beach begins, named Ka-ala-pueo.......... Ka-ala-pueo (Rallying-of-the-owls) was the last settlement near Makapu`u Point, and consisted of only a few fishermen's huts. Offshore lies Manana, the true name of the little gray volcanic island shaped somewhat like a crouching or recumbent animal and now popularly called "Rabbit Island." It has very little vegetation, and yet is the home of numerous wild rabbits [in 1972]. Just off its shore is a rock named Ka-ipu (The-cup) because of a depression in its top surface which held water coming from the waves breaking over it. On this rock there was another fishing shrine. (2)

Waimânalo in Hawaiian means "potable, or sweet water." Seasonal discharges from Puhâ Stream, which drains a primarily agricultural area, contribute to poor water quality along the shore. Net fishing for crabs, throw-netting, and bait-collecting occur where this stream enters the bay. The bay is influenced to a larger extent by Waimânalo Stream, which flows perennially, although there are several diversions in the upstream valley. The channelized mouth of this stream is estuarine.