The battle of the River Aesis (82 BC) was the first battle in the second year of Sulla's Second Civil War, and probably saw a Sullan army under Metellus Pius defeat part of the consul Carbo's army, under the command of one of his generals, C. Carinas.
We have two different accounts of this battle, one from Appian and one from Plutarch.
Both place the battle on the River Aesis, which formed the northern boundary of Picenum, where Pompey had inherited a large following from his father.
Plutarch places the battle early in the civil war, before Pompey had joined up with Sulla and agreed to serve under Quintus Caecilius Metellus Pius. In this account it was the third of three successes won by Pompey in this period. After raising a small army Pompey was faced by three Marian armies, led by C. Carinas, Cloelius and Junius Brutus. Pompey attacked Brutus's army, and defeated it in a cavalry battle. Next, the consul L. Cornelius Scipio Asiaticus was sent against him (placing the action in 83 BC), but Scipio's army deserted him as the two armies were closing on each other, forcing Scipio to flee. Finally Gnaeus Papirius Carbo attacked Pompey in person, and suffered a defeat in a cavalry battle on the 'Arsis'. Pompey defeated Carbo's cavalry and forced them to retreat into difficult ground, where they were forced to surrender.
Appian placed the battle in the spring of 82 BC, at the start of the campaigning season. This time Sulla's forces were commanded by Metellus, while the Marians were led by Carbo's lieutenant Carinas. The battle was fought on the banks of the Aesis, and lasted from early morning to noon. Carinas was defeated with heavy loss and forced to flee, and in the aftermath the area changed sides, joining the Sullans. In the aftermath of this battle Carbo (by then serving as one of the consuls for the year) came up in person and besieged Metellus in an unnamed location, but was forced to abandon the siege after his co-consul for the year, Marius the Younger, suffered a defeat at Sacriportus and was forced to take shelter in Praeneste. Pompey isn't mentioned at all in this account. Carbo retreated back towards Rome, pursued by Pompey. Although there is further fighting reported in the north of Italy, the main action after this took place in the area just to the north of Rome and around Praeneste.
It is of course possible that these accounts refer to different battles in the same area, one involving Pompey and Carbo in 83 BC and the second involving Metellus and Carinas in 82 BC. However Plutarch's account suffers from one serious problem. Both Plutarch and Appian report a similar incident of Scipio losing control of his army, but this time against Sulla at Teanum, just to the north of Capua. It seems unlikely that the same commander could suffer the same embarrassing fate twice in the same year. It also seems unlikely that Carbo would have spent his time dealing with a minor threat on the opposite side of the Apennines when Sulla was campaigning to the south of Rome.
The most likely version of events is that Appian has the context of the battle correct. In 82 BC Pompey was serving under Metellus, who as a proconsul was one of the most senior men to have sided with Sulla. Metellus had been sent north to establish himself in Cisalpine Gaul. The two consuls for the year split their efforts, with Marius facing Sulla and Carbo facing Metellus and Pompey. The first clash came on the Aesis, where Carinas, who was operating ahead of Carbo, clashed with Metellus and Pompey (who may have commanded the Sullan cavalry). Plutarch deliberately doesn't provide details of Pompey's actions in the north in 82 BC. Appian has him operating on the Adriatic, winning a victory at Sena Gallica, then moving towards Rome, fighting at Spoletium and the second battle of Clusium.