Ammianus Marcellinus Res Gestae 29.4.2

[2] But among many other cares, his first and principal aim was to capture alive by violence or by craft King Macrianus, ¹ just as, long before, Julian took Vadomarius; for Macrianus, amid the frequent changes in the policy followed towards him, had increased in power, and now was rising against our countrymen with full-grown strength. Accordingly, having first provided what the circumstances and the time demanded, and having learned from the reports of deserters where the said king, who expected no hostile move, could be seized, Valentinian threw a pontoon across the Rhine as quietly as his means allowed, lest anyone should interfere with the bridge while it was being put together.