Seneca post office was established in 1895 and named by postmaster Minnie Southworth for her brother-in-law, prominent Portland judge Seneca Smith. While early homesteaders moved into the valley in the late 1800s, Seneca only began growing in the 1929 when it became the northern terminus of the now-vacated Oregon and Northwestern Railroad, owned by the Edward Hines Lumber Company, which extended south to Burns. At that time, large-scale shipping of Ponderosa Pine logs from Seneca and the surrounding national forest began to the Hinessawmill in Hines. The company established a planing mill and railroad shops in Seneca, and it became essentially a company town. In 1940 Seneca's population was 275. Logging in the area began to decline in the 1970s, and the Hines company ceased operations of its lumber mills and railroad in 1984. The town was incorporated as a city in 1970 as lumber company control began to wane. According to the 1980 census, Seneca's population was 285. The estimated population in 2007 was 270.
Seneca has a semi-arid influenced subarctic climate with warm daytime temperatures in summer and cool to cold nighttime temperatures all year. Average high temperatures throughout the year are relatively mild for a subarctic climate. It is right on the border of the regular semi-arid continental climate with the fourth warmest month of the year straddling. Seneca experiences the coolest weather in Grant County and has the distinction of the coldest official temperature recorded in Oregon: in 1933.
As of the census of 2010, there were 199 people, 95 households, and 60 families residing in the city. The population density was. There were 128 housing units at an average density of. The racial makeup of the city was 99.0% White, 0.5% African American, and 0.5% from two or more races. Hispanic or Latino of any race were 1.0% of the population. There were 95 households of which 21.1% had children under the age of 18 living with them, 51.6% were married couplesliving together, 11.6% had a female householder with no husband present, and 36.8% were non-families. 31.6% of all households were made up of individuals and 15.8% had someone living alone who was 65 years of age or older. The average household size was 2.09 and the average family size was 2.53. The median age in the city was 52.5 years. 16.1% of residents were under the age of 18; 7.9% were between the ages of 18 and 24; 15% were from 25 to 44; 35.6% were from 45 to 64; and 25.1% were 65 years of age or older. The gender makeup of the city was 52.3% male and 47.7% female.
2000 census
As of the census of 2000, there were 223 people, 95 households, and 64 families residing in the city. The population density was 267.8 people per square mile. There were 115 housing units at an average density of 138.1 per square mile. The racial makeup of the city was 99.10% White, 0.45% Asian, and 0.45% from two or more races. Hispanic or Latino of any race were 0.90% of the population. There were 95 households out of which 29.5% had children under the age of 18 living with them, 58.9% were married couples living together, 6.3% had a female householder with no husband present, and 32.6% were non-families. 29.5% of all households were made up of individuals and 10.5% had someone living alone who was 65 years of age or older. The average household size was 2.35 and the average family size was 2.89. In the city, the population was spread out with 26.0% under the age of 18, 4.9% from 18 to 24, 25.1% from 25 to 44, 31.4% from 45 to 64, and 12.6% who were 65 years of age or older. The median age was 41 years. For every 100 females, there were 110.4 males. For every 100 females age 18 and over, there were 96.4 males. The median income for a household in the city was $27,333, and the median income for a family was $26,806. Males had a median income of $30,000 versus $23,750 for females. The per capita income for the city was $12,636. About 24.0% of families and 19.8% of the population were below the poverty line, including 22.5% of those under the age of 18 and none of those 65 or over.
Economy
The main industries in Seneca are cattle ranching and timber production.
