Final answer:
The Guadiana River forms part of the border with Spain and empties into the Gulf of Cadiz, with a unique ecosystem deeply affected by non-native fish species.
Step-by-step explanation:
The river that forms part of the border with Spain and empties into the Gulf of Cadiz is the Guadiana River. The Guadiana flows through an extensive catchment area on the Iberian Peninsula, characterized by a Mediterranean climate, which contributes to its high seasonal and interannual flow variability.
This river is also significant for its ecological status, as it supports a native fish fauna with high endemicity and conservation concern but is also highly invaded by non-native fish species, impacting its biotic integrity.