The purpose of this project was to determine the historical and contemporary flora of the Arroyo Seco. This was accomplished by researching and compiling historical documentation relating to the botany of the Arroyo Seco from various sources such as the Herbarium collection at Rancho Santa Ana with thanks to Valerie Soza, The Flora of Pasadena and its Vicinity written by Alfred MaClatchie in 1895, the CALflora database and other sources. In addition to collecting this data, plants were collected throughout the summer at Switzer Picnic area and along the Gabrielino Trail to help establish the species composition of the current flora for the basis of comparison. This involved collecting, pressing and identifying plants thanks to the help of Dr. Gretchen North. In order to conduct this investigation, several weeks were spent obtaining background information. This included, but was not limited to, information regarding the Hahamongna tribe that once occupied the area, the invasion and conquest by the Spanish, the building of the freeway and dams and the subsequent changes in hydrology, and the ecology of the area provided by Dr. Elizabeth Braker and Dr. Marcus Renner. With this information, we can tell whether species composition has changed over time or has remained relatively stagnant. For example, the implementation of the cement flood channels of the lower Arroyo Seco which began in 1938 may have caused fragile species to become locally extinct, while allowing other invasive species to take over the area.