Parts of the continental shelf extend far into the bay, resulting in fairly shallow waters in many areas and thus the rough seas for which the region is known.

The International Hydrographic Organization defines the limits of the Bay of Biscay as "a line joining Cap Ortegal (43°46′N 7°52′W / 43.767°N 7.867°W / 43.767; -7.867) to Penmarch Point (47°48′N 4°22′W / 47.800°N 4.367°W / 47.800; -4.367)".

[4] The main rivers that empty into the Bay of Biscay are Loire, Charente, Garonne, Dordogne, Adour, Nivelle, Bidasoa, Oiartzun, Urumea, Oria, Urola, Deba, Artibai, Lea, Oka, Nervión, Agüera, Asón, Miera, Pas, Saja, Nansa, Deva, Sella, Nalón, Navia, Esva, Eo, Landro and Sor.

In late spring and early summer, a large fog triangle fills the southwestern half of the bay, covering just a few kilometres inland.

These depressions cause severe weather at sea and bring light but very constant rain to its shores (known as orballo, sirimiri, morrina, orbayu, orpin or calabobos).

[5] The Gulf Stream enters the bay following the continental shelf's border anti-clockwise (the Rennell Current), keeping temperatures moderate all year long.

The main cities on the shores (or close to) of the Bay of Biscay are Bordeaux, Bayonne, Biarritz, Nantes, La Rochelle, Donostia-San Sebastián, Bilbao, Santander, Gijón and Avilés.

The southern end of the gulf is also called "Mar Cantábrico" in Spanish (Cantabrian Sea), from the Estaca de Bares, as far as the mouth of Adour river, but this name is not generally used in English.

[citation needed] The Plaiaundi Ecology Park is a 24-hectare coastal wetland lying where the Bidasoa River meets the sea in the Bay of Biscay.The nature of Plaiaundi consists of a wide variety of flora (visitors view them mainly in the spring) and fauna (visitors with binoculars arrive all during the year, because of the birds' migratory habits).

Volunteers and employees of ORCA regularly observe and monitor cetacean activity from the bridge of the ships on Brittany Ferries' Portsmouth to Santander route.