This is the default behaviour of for \chapter (and \chapter*) in both book and report document class, so you're doing nothing wrong. This default length is 50pt.
You could use the etoolbox package to remove (or modify) the spacing above the chapter headings. Here's a minimal example that will work with either book or report document class:

\documentclass{book}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
\patchcmd{\@makechapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter head
\patchcmd{\@makeschapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter* head
\makeatother
\begin{document}
\chapter{A chapter}
\chapter*{Another chapter}
\end{document}
\patchcmd searches for \vspace*{50\p@} in both \@makechapterhead and \@makeschapterhead and replaces it with nothing, thereby removing the space. If you want to add a little space, you can insert something different as the replacement text. For example
\patchcmd{\@makechapterhead}{\vspace*{50\p@}}{\vspace*{20pt}}{}{}%
\patchcmd{\@makeschapterhead}{\vspace*{50\p@}}{\vspace*{20pt}}{}{}%
will insert 20pt instead of the default 50pt.
showframe was added to show the frame of the text block and was merely used for illustrative purposes - you can remove this.
An alternative to this would be to use the titlesec package. However, in order to modify the \chapter spacing (via \titlespacing{\chapter}...), you are also required to modify the \chapter format (via \titleformat{\chapter}...). See the titlesec documentation for more information.
scrreprtis really strange. Usually you don't want so much whitespace before a chapter, at least if you have a limit of totally available space. – TomM Dec 26 '13 at 13:33