Drainage networks on the accretionary eolian mantles of 11 K-Ar-dated lava flows in the Cima volcanic field and 8 K-Ar-dated flows in the Lunar Crater volcanic field show several related and progressive changes with flow age. If it is assumed that these changes with flow age reflect changes through time, they provide a useful perspective on the early stages of drainage-network development. These drainage networks grow in a manner that is consistent with the qualitative model proposed by Glock. After accumulation of an eolian mantle (requiring 0.1 to 0.2 m.y.), master drainages extend to all parts of a flow. During this period of elongation, which lasts for ∼0.2 to 0.3 m.y., drainage density (D) and link frequency (F) increase rapidly whereas the value of Shreve's kappa (κ) declines. After ∼0.4 m.y., elaboration replaces elongation as the principal mode of network extension and remains predominant throughout the remainder of the 1.1-m.y. period of record. Elaboration proceeds mainly by the formation of short tributaries along existing streams and by the creation of short streams along flow margins. During the early phases of elaboration, D and F attain a condition wherein the number and lengths of channel links are approximately adjusted to one another, and κ reaches a minimum value of ∼0.75. Thereafter, the continuing addition of short tributaries to existing streams causes D, F, and κ to increase slowly as the networks evolve toward maximum extension.