Mainauen bei Sulzbach und Kleinwallstadt, Protected nature reserve in Bavaria, Germany.

Mainauen bei Sulzbach und Kleinwallstadt is a nature reserve on the right bank of the Main between Sulzbach am Main and Kleinwallstadt. It protects a 50 to 400 m wide, roughly 5 km long strip of riparian woodland, meadows and old river structures, the last near-natural floodplain on the Lower Main. The reserve is split by the St2313 road and the Natorampe access path.

Until the early 19th century the Main's banks were unfixed and often swampy, with shallow, seasonally fluctuating water; boats were towed upstream by horses. Mid-19th-century mid-water correction set a minimum fairway depth of 90 cm and width of 26 m using groynes, and towpaths were hardened. From 1920 the river was dammed into a large shipping channel, and the floodplain survived because the 1876 Maintalbahn cut it off from Sulzbach and floods blocked settlement; it was designated a nature reserve on 16 May 1994.

The name "Kleiner Gries" marks the 550-metre-long, up-to-70-metre-wide strip near Kleinwallstadt, the reserve's most sensitive zone A, where entry is forbidden. In the Kohlplatz to the north, the Sulzbach stream was rerouted during river regulation and now enters the Main about 150 metres downstream of its original mouth. In spring much of the Kohlplatz is covered by a dense carpet of wild garlic.

The reserve lies on the right bank of the Main between Sulzbach and Kleinwallstadt, flanked on the east by the St2309; the St2313 to the Main bridge and the access road to the Natorampe cut it into two parts. Zones A and B, including the 550 m long Kleiner Gries, are closed to entry.

The 'Kleiner Gries' near Kleinwallstadt is a 550 m long, up to 70 m wide strip in protection zone A, the reserve's most sensitive category, where entry is forbidden. Its roughly 70 m oxbow connects to the main river only through the downstream groyne field, and dense willows, tall herbs and reed beds surround it.