the Sierra ranges from semi-arid in the north and subhumid in the south, with average annual rainfall ranging from 800 mm in drier north up to 3000 mm in the south. The eastward-facing slopes of the Sierra generate orographic precipitation. The Mexican Plateau is in the rain shadow of the Sierra Madre Oriental, and has an arid to semi-arid climate, with an average annual rainfall of 300 mm. The principal source of rainfall is easterly trade winds, which strengthen from late June to August. Tropical cyclonic storms can bring additional moisture from August through October. August is generally the highest rainfall month. Rainfall is generally low from November to mid-June, except for occasional mid-latitude storms coming from the north during the winter months. The volume of water carried by the river is highest during late summer and early fall, and is generally low from November through June. Ecology The Pánuco River basin is rich in fish. There are almost 100 fish species, including a few that were introduced. There are many endemics: six Nosferatu cichlid species, five Tampichthys minnows, nine "northern swordtails" (genus Xiphophorus), three Gambusia species, two Ictalurus catfish, the bluetail goodeid (Ataeniobius toweri), dusky splitfin (Goodea gracilis), relict splitfin (Xenoophorus captivus), pygmy shiner (Notropis tropicus), checkered pupfish (Cualac tessellatus), broadspotted molly (Poecilia latipunctata), Tamasopo cichlid (Herichthys tamasopoensis), Calabazas shiner (Notropis calabazas) and fleshylip buffalo (Ictiobus labiosus). Additionally, a couple of still-undescribed species are known from this river basin. A few of the endemics are seriously threatened. See also Tampico Bridge List of longest rivers of Mexico References Sources and external links "Panuco", Freshwater Ecoregions of the World "Tales of Southern Rivers", by Zane Grey , 1924, True story of descent of Santa Rosa and Panuco Rivers Rivers of San Luis Potosí Rivers of Tamaulipas Rivers of Veracruz Drainage basins of the Gulf of Mexico Tampico