The battle of the Glanis River (82 BC) saw Sulla defeat a force of Celtiberian cavalry that had been sent to help the Consuls in their attempt to resist his invasion of Italy (Sulla's Second Civil War).
At the start of the fighting in 82 BC the Consuls for the year, Gnaeus Papirius Carbo and the younger Marius, had split their forces. Marius went south to face Sulla, while Carbo moved north to face Metellus Pius and Pompey. After an initial defeat on the Aesis River, Carbo besieged Metellus at an unknown location, but Marius didn't do so well. He suffered a heavy defeat at Sacriportus, and had to take refuge in Praeneste, which was then besieged by Sulla's forces. Carbo was forced to return south, but he never reached Rome. From Praeneste, Marius sent instructions to Rome for a massacre of his opponents. Soon after this, Sulla was able to occupy the city without a fight. Carbo must have been moving south while this was going on, as by the time Sulla had finished in Rome, Carbo had reached Clusium (Chiusi), just over eighty miles to the north of Rome.
The battle of the Glanis River was the first of three encounters presented as having happened in a fairly short period in Appian. The Glacis (or Clanis) rose to the west of Arretium, flowed south past Clusium and into the Tiber just to the south of Volsinii. Sulla's logical route from Rome would have taken him up the Via Cassia, which led from Rome to Volsinii and then to Clusium.
The Consuls had clearly retained some support outside Italy. The praetors in Spain (where the Marians later made their last stand under Quintus Sertorius), sent a force of Celtiberian cavalry to join Carbo's army. This force appears to have arrived after Carbo's retreat from the north of Italy and were now perhaps acting as a cavalry screen on the Glanis, somewhere to the south of Clusium.
The Celtiberian cavalry ended up getting into a cavalry fight with Sulla's men on the banks of the River Glanis. About fifty of them were killed in the battle. Another 270 then deserted to Sulla, one of a series of desertions that blighted the Marian war effort. Carbo's response was to massacre the rest of the Celtiberians. Appian doesn't say how many men were involved in this massacre, and gives two possible motives - anger because of the previous desertion, or fear that the remaining troops would copy their countrymen.
The second battle took place at Saturnia, a spa town well to the west of the Glanis, on the Via Clodia. Appian has this happening at the same time as the massacre of the Celtiberians, and it would seem unlikely that Sulla himself would have moved west across difficult country from the Glanis to Saturnia and then back again to fight in the third battle of the sequence, at Clusium, so this second battle probably involved a detached part of Sulla's army that had been sent up the Via Clodia instead of the Via Cassia.
The final battle took place at Clusium, on the Glanis. This was a day long battle, but ended inconclusively and Sulla had to withdraw (Clusium appears to have remained as a Marian base until almost the end of the war). However Carbo suffered a setback at Spoletium (Spoleto, on an eastern branch of the Via Flaminia), 40 mile to the east/ south-east of Clusium. This saw Pompey, who had clearly followed Carbo south, defeat Carinas, one of Carbo's generals.