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Assessing timescales for formation of the ancient Martian valley networks is key to interpreting the early climate on Mars. We determined the likely formation times for seven of the largest ancient valley networks on Mars using the Darcy-Weisbach equation for average flow velocity, three different sediment transport models, and a range of input parameters to encompass possible formation conditions. With runoff rates similar to intense storms in arid regions on Earth, the minimum formation timescales of these Martian valley networks range from 10(5) to 10(7) yr, depending on the specific valley network. Shorter formation timescales require hurricane-scale flows that, if minimized with assumptions of continuous formation unlike even terrestrial rates, could complete large valley network formation in as little as 200 to 5000 yr, though this is not the preferred interpretation. While these results do not support impact-induced climate change as the sole mechanism for creating precipitation sufficient to incise the large valley networks, neither do they extend the amount of time required to form the networks beyond their range in age, consistent with hypotheses that valley incision was constrained to a relatively short period of Martian history near the Noachian-Hesperian boundary, approximately 3.6 to 3.8 Ga. (C) 2011 Elsevier B.V. All rights reserved.